Start with two pots: Get two pots, one large and one small. Fill the large one with water and put the small pot in it. This keeps the temperature in the small pot from going above the boiling temperature of water, and that means your butter won't burn.
Measure out the weed and butter: A good guide to follow is one stick of butter for every eighth of marijuana (much more if the weed is stress or shwag). That would mean 8 sticks of butter for every ounce. If you want stronger butter add more weed.
Grind the Weed: Grind it up the best you can. It will make it easier for the THC to be released into the butter.
Boil the Weed: Put water in the large pot. Put the butter and the weed into the small pot. When the water starts boiling in the large pot, put the small pot in it. Be careful! Leave for 20 minutes, and stir occasionally.
Strain it: There are all sorts of methods for straining weed. Use whatever you have. Cheese cloth is the best, but metal strainers and coffee filters work too. Just make sure to get as much butter as you can. You don't have to throw the extra weed away. Bake the leftover weed into brownies, they taste a little weed-y but they still do the trick.
Now you have weed butter. Use it as you will. Happy cooking.