There are lots of webpages out there that explain how to make it, where to get the materials to do it and where to get rid of the by-products.
Your better off Googling it.
In a Nutshell: you take grease put a chemical(lye?) in it, the soap separates out, you wash it with menthonal, rinse it with water, then something else happens, I forget????
People have been making it for quite some time now. IF you can find a good reliable supply and you're willing to put the hours in, it can be worthwhile. I looked into it a while back and concluded that it just wasn't worth my time to fight over supplies (typically from restaurants) with others that would pay for it, and then the time to spend actually brewing, and then disposing of the glycerine byproduct. I'm sure for others though it's worth it. Time was once where the used vege oil could be gotten for free or even for pay(!) but there's plenty of people chasing it now. The other thing is that my diesel truck is under warranty and unless I blended it to 20% or less with diesel, I'd void the warranty.
Cheers,
Neil.