The traditional way of using cannabis is to inhale the smoke from a joint. While there's a lot to be said for this method, in particular the fact that it's relatively cheap and simple, traditional isn't always best.
Some people simply do not like the act of smoking, possibly because it irritates the lungs. Others want ways to ingest cannabis without the smell or taste associated with smoking. Others simply want to use their weed as economically as possible. These are all reasons for turning to liquid cannabis or eliquid. This is essentially cannabis, infused in a liquid, which is then used with a vaporizer. Here's how you make it.
Chop about 6G of buds as finely as possible. You may find using a powered or manual chopper better than a knife. Put tin foil on a baking tray and put them in the oven on the lowest heat for about 15 minutes. Put your buds into a jar and put in enough high-proof grain alcohol to cover them. As you're going to be vaping the liquid rather than smoking it, you don't need to use top-quality vodka for this, but you do need to use a decent brand of alcohol rather than, say, cheap rubbing alcohol.
Create a bain marie, in other words, put enough water in a pot for you to be able to stand your jar in it comfortably. Keep stirring the contents of the jar and allow the alcohol to evaporate.
You may wish to give some serious thought to ventilation while you are doing this. The mixture will get darker as the THC is transferred into the alcohol. When it is dark enough to suit you, remove the jar and top up the alcohol to its original level (or thereabouts). Filter the liquid to remove the buds, pour it back into the jar and reheat it until there is 5ML of liquid left. Then add 20ML of propylene glycol or vegetable glycerin.