alright. this process will work for butter, but i typically do it with coconut oil because you can bake with it or make salves or other topical ointments, too.
I tend to go way heavy with my measurements because the people I give the shit to tend to have arthritis, MS, or cancer. So, I'll take ~6 heaping cups of oil to a quarter pound of shredded, dry, decarbed trim, popcorn buds, and larf. I'll put that in a ceramic bowl, that I then place in my crockpot filled with water and let it cook overnight. A proper double boiler would work better, like what you would use for making chocolates and whatnot. Longer the better, you don't want the temperature to exceed 180F if you can avoid it, 160F is more an ideal temp imo. I then filter the oil out into another bowl using cheesecloth. Then, filter again into another container, again with cheesecloth. Then, I'll put it into the final containers, usually the little 4oz mason jars.
Anticipate losing at least 25% of your starting butter/oil to the material. You simply will not be able to extract it from the plant material.
The next time I do this, I'll be making a shit ton of QWISO first then will use that to make the oil using the same process as to avoid losing so much oil to the material and being about to dose it better. Typically I estimate ~250mg THC per tablespoon, maybe a bit more for the topicals.