Get a san pedro or peruvian torch cactus (they are legal). Supposedly they are sold at home depot and garden center places but I ordered one online. The peruvian torch is more potent than the san pedro. If using the san pedro use a 12 inch segment per person. I have never used the peruvian torch.
step 1: cut the cactus into segments and remove the white core (this can be discarded)
2: take what is left and put it in a blender spines and all
3: after blending put the contents in a cooking pan and cook on a medium heat. Basically all you have to do is consume the liquid from this so boil it for a while and then strain it through a cheese cloth, strainer, old t shirt or whatever. You can repeat the cooking process with the leftover pulp if you want to get more juices out of it.
4: Once you have strained it you can consume the liquid as is but I wouldn't reccomend it (it is very bitter). Instead I would reccomend boiling it longer on a medium heat. The longer you boil it the more concentrated it will become. I found that using the liquid to make jello is a much more desirable way to consume the final product. It doesn't taste good but it is not unbearable and once you have consumed it, it will not make you sick at all. It takes a while to take effect ( about 2-3 hours). It is a very spiritual and enlightening experience. Having some weed will also kick things up a notch. The cactii are not illegal but the final product is so don't tell the police you are making it. Enjoy