Cannabis oil is extracted by putting the cannabis in a basket and then reciculating a solvent through the cannabis. THC is not water soluable, therefore a solvent must be used. The solvent must evaporate COMPLETELY. Popular solvents for this use are ether or pure alcohol. Older systems used to work like the old style coffee percolator pots. You put the solvent in the pot, grass in the basket, and use a 100W light bulb under the pot for a heat source. The problem with this is that many people were blown up when the solvent vapors ignited. This often happened when people accidently spilled liquid on the hot light bulb. Not pleasent. A newer method using cold solvent by using a small tubing pump which circulates the solvent and generates less vapor. Either way, you want a high degree of ventilation to avoid an explosive atmosphere! After the cannibis is 'bleached' (it turns white or very pale), the solvent is evaporated, leaving the oil which was leached from the weed. The amount of yeild depends upon the parts of the plants used (even stems and fan leaves yeild a small amount of oil!), and the plant itself. The more resinous a plant, the more oil obtained. It is impossible to make an exact determination of quantity until you actually do it. Typically an ounce or more is required to obtain 1-3 cc's of oil -- and that's on a good day with great weed.