Making Cannabis oil
Cannabis oil is becoming the most favoured method of using cannabis medicinally. There are many ways to make cannabis oil and different methods have different advantages depending on the means and medical applications.

Decarboxylation simplified
The term decarboxylation sounds complex, but the process of decarboxylation is very simple.
Heat the cannabis in the oven at 120º Celcius for an hour, and it's decarboxylated, it's as simple as that. This converts the THCA cannabinoid into THC, CBDA into CBD, etc, etc. It removes the carbon atom and allows the cannabinoids access to the cannabinoid recptors CB1 and CB2.
I suggest doing this prior to making the oil, then you are assured of decarboxlation.

Rick Simpson Oil (RSO)
The main aspect that defines Rick Simpson Oil is the use of a rice cooker to boil off the solvent. Usually either Naptha or Isopropyl alcohol. Rick Simpson oil is promoted as a cancer treatment so full decarboxylation is required to maximise its efficacy as a cancer treatment.
Although Rick Simpson primarily promotes naptha, with Isopropyl alcohol another solvent he recommends, more people are substituting these inedible solvents with edible solvents like Grain alcohol.
Regardless of solvent used it is vital that Rick Simpson oil is properly cooked and while this can be achieved with the rice cooker alone, often the oil is only partially cooked from the rice cooker stage and either requires additional cooking in the rice cooker (with careful monitoring of temperatures) on a coffee warmer (which can often take many hours to finish) or in an oven or other heating device. Though doubt has been raised over the efficiency of coffee warmers for decarboxylation and higher temperatures are advised for rapid and full decarboxylation.
Optimal decarboxylation temperatures are between 110 and 130 degrees Celsius and usually take between 30 minutes and an hour to complete to a point that all cannabinoid acids have converted to their neutral counterparts.

The Rick Simpson process involves mixing the plant material, preferably high grade bud material, with a solvent.
Solvents like naptha are more selective than alcohol and will strip less chlorophyll but chlorophyll content of alcohol extractions can be minimised by limited exposure to plant material and using high grade bud material rather than leaves and trim. Freezing both solvent and plant material prior to extraction is also a common practice among oil makers for this purpose.
Approximately 5 minutes of soaking and mixing bud material, ideally crushing it up thoroughly, is sufficient to strip the majority of cannabinoids with many utilising a second wash to get most of the remaining resin from the plant material. A third wash will perhaps strip final amounts but is often deemed unnecessary as most will be taken from the first two washes and these additional washes are more likely to increase chlorophyll content.
After using a sieve to filter the larger pieces of plant matter from the solvent oil mix you then need to filter more thoroughly, ideally using coffee filters which are ideal for this purpose, with many recommending that solution is filtered twice to maximise purity of end product.

Once the oil solvent mix has been properly filtered it is then poured into a rice cooker and boiled off. This needs to be done in a ventilated area, preferably outside, and away from any red hot elements or open flames that could ignite the fumes from the solvent.
It is advised not to fill the rice cooker to the brim as the mix will spill over as it boils. As the last of the solvent boils off it is advised to gently swirl the contents to help release last traces of solvent and keep heat evenly dispersed.

At this stage some partial decarboxylation will have occurred from the heat involved in boiling off the solvent but additional cooking will be required to complete this process. Rick Simpson suggests a coffee warmer as a good example of a gentle heating device and while it maybe true that this can complete decarboxylation, given enough time, higher heating devices are more suitable. An oven is not as precise with the temperatures but monitoring with a thermometer can ensure that the cooking temperatures stay within the optimum range of 110 to 130 degrees Celsius. Although the time taken for full decarboxylation at these temperatures is usually within an hour, it is recommended to monitor the activity of the oil as an indication that decarboxylation is complete.
When there is no activity, in the form of tiny bubbles and pin prick pops, on the surface of the oil (while the oil is at the optimum temperature range) then the decarboxylation process is complete and it is advised to stop cooking to minimise degradation.
When activity stops at temperatures above 110 degrees Celsius then decarboxylation is complete, any residue solvent will also be well purged and oil will be ready for use. Activity is still observable at the lower temps involved with the coffee warmer (usually between 60 and 80 degrees Celsius) but will be less aggressive and obvious.
When the oil cools it becomes sticky and less runny, often varying from a marmite like consistency to a thick grease. It can be heated to a softer and more of a runny consistency for the sake of sucking up into syringes or pouring into containers.

BHO (Butane Honey Oil)
Butane Honey Oil is popular among recreational users who make raw oils for the purposes of smoking. It is a cold extraction method so decarboxylation during the making of it is minimal or usually not at all, so will require cooking to be “activated” and ready for ingestion. Though the lack of decarboxylation is often preferred for extracts that are to be smoked or vaporised as raw extracts retain more terpenes which give it more flavour and smell. The lack of decarboxylation also makes such extracts ideal for those who wish to ingest the raw cannabinoid acids rather than the neutral cannabinoids.
Butane extractions involve pumping butane through a container filled with cannabis plant material to strip the THC and other cannabinoids, which can then be collected in an oven suitable dish as the butane cannabis mix drips down. The butane will mostly evaporate at room temperature and minimal heat will be required to purge it fully. Though, as mentioned, more thorough cooking will be needed to cause decarboxylation. More experienced makers will often use vacuum purging techniques to remove trace residues.
Because BHO is usually produced for smoking, which causes decarboxylation anyway, the oil is usually raw with minimal to no decarboxylation achieved, minus partial decarboxylation that occurred as plant material is dried prior to extraction process.
Though there are various methods for making Butane Honey Oil the general rule is to pack cannabis plant material into a container, making sure that the container is made of a suitable material that will not leach chemicals. The container will need to be sealed apart from two holes either end, one for pumping the butane into and another for the butane oil mix to escape out of.
The butane will drip onto a dish, preferably oven safe, and most of the butane will begin to evaporate almost immediately at room temperature (though additional purging will be required to get rid of traces of butane).
Unlike RSO which is at least partially cooked from the rice cooker process of boiling off the alcohol, BHO will be mostly, if not completely, raw as little to no heat is required to evaporate the butane. This is not an issue for people who wish to smoke or vaporise their oil as decarboxylation (conversion of raw cannabinoid acids like THCA to neutral cannabinoids like the psychoactive THC) will occur instantly from the heat involved in smoking or vaporising it but is an important factor in anything that is ingested or applied through suppositories or topical applications.
If the oil is being made for ingestion, unless you purposely wish to avoid decarboxylation, you must cook the oil sufficiently.
Apart from being almost completely raw prior to additional cooking (oppose to the partially cooked RSO) Butane Honey Oil will also be much clearer and lighter coloured. It is referred to as honey oil as it is more honey like in appearance. The reason for this is because Butane is far more selective as a solvent than alcohol or even naptha.
High grade RSO is often more golden when spread very thin but in larger amounts will appear brown to black in colour, sometimes with a slightly reddish tinge. Lower grade oils made from leaves or trim, or oils that have been soaked in alcohol for a long time will have even more chlorophyll and will even have a green colour to them.
Butane oil has virtually no chlorophyll and as a result of this the oils produced are much purer and golden in colour. This is because butane is one of the most selective of the more attainable solvents. While Butane Honey Oil is relatively easy to make for experienced producers it is potentially expensive to produce in large amounts and, like RSO, is potentially very dangerous to produce and is not recommended for novice oil makers.
Shona Banda Oil
This method is well known through the work of Shona Banda who used a vaporiser to produce small amounts of cannabis oil. Basically the method involves using a vaporiser with a glass dome to capture the vapour. The hole used for sucking the vaporised cannabis is sealed so that the vapour cannot escape and it is then allowed to cool. As it cools the vapour condenses and sticks to the bowl and the resulting oil can then be scraped out while it is still warm and soft enough to do so.
The vaporiser method is very cheap and very easy but is also very time consuming and only small amounts can be made at a time. The vaporiser Shona used was a VP500, a very cheap vaporiser which usually costs less than £40. But only small amounts of cannabis can be vaporised at any one time.
The main advantage of the oil made using Shona Banda’s method is, apart from how easy and safe it is to make, is that it requires no solvent. The cannabinoids are vaporised off the plant material and as the vapour cools it condenses and collects in the form of oil. The oil is very pure and decarboxylation is already fully completed by the initial vaporising process. So while oil made this way will be time consuming and gradual the oil made from it will be very high quality and decarboxylation will be complete. Such oils will also retain more terpenes as they are unable to escape and are trapped along with the cannabinoids.
To make larger amounts of oil a larger, purpose built, custom made, vaporiser would be needed.

Olive Oil or Butter extractions
Oils and fats can also be used to make cannabis extracts. Though the use of butter would give you cannabis butter, otherwise known as cannabutter, oil extractions like olive oil extractions would give you an oil that is very diluted (as you cannot boil off the olive oil after extraction) these extraction methods are far more safe and user friendly but will not result in a concentrated oil.
If you wish to complete decarboxylation it is advised to cook the cannabis prior to making the butter or olive oil cannabis oil mix. The reason for this is simply that it is difficult to apply the necessary cooking temperatures to the cannabis once it is infused in Olive oil or butter, though partial decarboxylation is inevitable as heat is often used to help fuse the cannabis to the olive oil or butter.
Cannabis can be infused into the oils, fats or butters used without the application of heat, for those who wish to make raw extracts, but this would take days, even weeks to properly extract and with out testing it is hard to gage how efficient such cold infusions really are.
THC and other cannabinoids absorb into fats well and when extractions are made for the purposes of ingestion then fusing them into fats, like butter, are considered a good way to extract cannabinoids from the plant material and a good way to ingest them as digestion will be efficient and cannabinoids will be well absorbed by the gut.

Other methods and general summary
There are other extraction methods like Co2 extraction for example but these methods usually require more sophisticated techniques and facilities that most people do not have access to.
Regardless of which method you use for cannabis extraction the important points to remember are the need for pure solvents that will evaporate cleanly, if you are using a solvent, and if you are wishing to maximise the neutral/activated cannabinoids like THC or CBD then remember that proper cooking times and temperatures are required to complete full decarboxylation of the raw cannabinoid acids like THCA and CBDA.
It depends on your requirements and facilities as to which oil making method is most suitable. Oil or fat extractions are ideal for those who wish to avoid solvents but will produce a diluted extract. Vaporising methods are safer and easier but usually make smaller amounts at a time while methods that utilise solvents like RSO or BHO are more suitable for making larger amounts of more concentrated oils, but are potentially dangerous and require great care and caution. Alcohol is a cheaper solvent and is very effective at stripping cannabinoids and making a potent oil. Isopropyl alcohol is relatively cheap and a relatively easy to obtain solvent in pure form (99.9% is ideal because of the lack of water content, as well as lack of denaturants). Isopropyl alcohol, while not suitable for drinking, is also relatively low in toxicity compared to other solvents, like naptha or hexane, while grain alcohol (Ethanol) is often preferred by many oil makers as it is even less toxic and actually suitable for consumption as it is basically drinking alcohol (though properly made oils should have little to no residue in the finished product regardless of solvent used).
Butane is one of the most selective solvents and produces a much purer oil with virtually no chlorophyll. Also for those who wish to avoid decarboxylation butane is ideal as it involves a cold extraction process with little to no heat required. RSO still needs additional cooking, after extraction, to complete full decarboxylation but the heat involved in boiling off the solvent in the rice cooker will cause some decarboxylation so is unsuitable for those who wish to keep their oil raw for the purposes of utilising raw THCA and CBDA rather than THC and CBD.
The important things to remember when making cannabis oil is to take all necessary safety precautions during the oil making process, especially when cooking off solvents. Make sure solvents are pure, so they will evaporate cleanly, and be sure to filter well to ensure purity of product. If decarboxylation is required then care should be taken to cook oil sufficiently to achieve this and finished oils should be stored properly (ideally in sealed containers away from direct sunlight) to avoid degradation.