I have made it quite a few times... failed and succeeded and have decided to make a little tutorial. Though most people probably already know how to do this... I am making this for those that don't.
You can use almost anything to make butter... from good buds to leaves and stems. I prefer the leaves and stems because the buds are good for smoking (if you have buds... try smoking them, its fun!

).
This is how to make butter for those who dont own a crock pot.
Step 1:
Pull out your leaves/trimmings/buds
Step 2:
Pull out the butter (I used 3 cubes for this batch)
Step 3: melt down the butter in a small pot (unless you are making a lot) directly over the flame
Step 4:
Add your MJ.
Step 5: Once the butter is melted and the grass is in it, move the pot into a slightly bigger pot that has enough water in it to cover the smaller pot atleast half way up the sides, but not too high as you dont want water boiling up into your butter.
Step 6: Lower the flame and cook the butter for many hours (i do it for atleast 4 hours... but the longer the better) Be sure to stir it every once in a while and add hot water to the double boiler when needed.
Step 7: Take a reusable coffee filter (a cheese cloth or a regular coffee filter might work) and pour the butter through the filter. Smoosh the left over leaves and stuff to get as much butter as you can.
P.S. I recommend making extra creme brules the night before to leave in the sink for this step.
Step 8: Bask in your glory (by pouring the butter into a tupperware container)
Step 9: close and freeze the butter
Or if you want to bake with it right away, you can just pour it directly into your recipe. Here are a couple results from my trials... the one on the left wasn't cooked as long, but it was made with buds and the buds were left in it... the one on the right was made with leaves and trim and a little keef and was done like this tutorial. The green one on the right was stronger.
Hope this helps.