Here's how I make my oil. Sure, I've heard a lot of people say acetone may not be the best solvent and whatever. I have made iso oil and I have also used butane before. They all work but for some reason I prefer to use acetone. But butane oil is definitely cleaner, i just think it's easier to make the acetone oil and frankly I do it mostly while being really stoned.
So here's what i do:
Grind up the weed and stick in the freezer. I've read that when it's frozen the goods will fall out easier, i don't know if this really makes that much of a difference but it doesn't hurt. At least, I don't think it does.
Get your jar, or whatever container you'll be mixing in, and throw in your frozen weed. Then pour enough acetone to cover the weed and a little bit more.
Now, I've read different teks and I've heard people say different things about how long to let the weed and acetone in the jar. I've heard some people say a couple of hours, over night, several days and so on. I personally let them mix for maybe 5 minutes per wash, while really shaking the hell out of it. So, i pour some acetone in, shake vigorously, pour out the acetone and repeat the whole thing with fresh acetone. I do it two or three times more and then i'm set.
Now you evaporate the acetone. There are several ways for you to do this. Do whatever you want but use some common sense, open flames are not a good idea and neither is just letting it sit under the sun. I usually just put all the acetone in a large pyrex container with significant surface area, the more the quicker the evaporation will be. Once the acetone is in this container, i put it over an electric heat source (this may be a stove, a heat pad, even a mug warmer could work) and place a fan over it. The combination of heat and circulating air will make the acetone evaporate in a timely fashion.
So you have evaporated all the acetone but now your oil is spread all over the container. It's sticky as hell and a total bitch to work with, isn't it? But do not worry, there are several ways to get your product out of there. Something i did a couple of days ago and i hadn't done before is stick the whole thing in the freezer. Freeze it completely and then just scrape it off with a razor blade or whatever you want. Freezing it makes the whole process exponentially easier, so i really recommend that. You could also just scrape it off without freezing but, like I said before, it's totally worth it. Another option would be to pour in a minimum amount of acetone and redissolve the oil in the acetone. You want to use just the amount of acetone, then your pour it in the final container adn evape off the acetone right there. That's why you want to use the smallest amount of acetone possible.
So there you go, you now have a fresh batch of oil. It can be used as is, just make sure you done apply a flame firectly to it. You'll notice how it will burn up rather quickly and that you need a minimum amount to get really high for several hours. I love this stuff.
Extra Step: This step isn't really necesary but I love the end product. No great science involved here, simply mix your oil with some kief or extremely ground up weed. This will leave you with a much more manageable form of hash. You can just roll it into a ball and rip off a small chunk to smoke when needed.