Also called "pre-baking," this step leads to stronger bang for you bud. Marijuana is filled with many compounds, some of which are medicinal/psychoactive, and some of which are not. You can get rid of the not so useful ones if you decarboxylate the weed first, which simply means baking it at a low temperature. This vaporizes compounds like THCA, which doesn't get you high, leading to a stronger butter. You can also use shake, leaves, or a mixture of buds and leaves if you'd like. You want a nice, low temperature. While a higher temp will remove the compounds faster, it will also cause wanted compounds, like terpenes (which give each strain it's unique aroma and flavor) to leak out too. 240°F is a good, safe temperature. You want smaller pieces of bud, roughly 1/2" or less in diameter, though exact measurements aren't needed. Use a baking sheet just big enough to contain all of the weed. Empty space will heat up the pan quickly, which causes uneven "cooking." When done, the weed will be a light brown color and will crumble easily in your hands. Don't grind it into a powder -- you just want it to be in small, manageable bits, like small pieces of gravel. You can use an herb grinder or simply pulse it quickly in a blender or food processor. This will remove some of the chlorophyll, dirt and fertilizers from the plant matter which can cause a "grassy" taste in your butter. THC and other cannabinoids are not water soluble, and will not be affected by the soak. When done, strain out the water with a colander and pat dry with paper towels.

Summary:
Know that, while not necessary, decarboxylating is a simple process that makes the most of every bud. Pre-heat the oven to 240°F (115°C). Hand-crumble the weed into in a rimmed baking sheet, spreading so there are no holes or gaps. Bake the buds for 30 minutes or so, mixing it every 10 minutes to prevent burning. Grind the bud until it is a course mixture, with recognizable buds. Optionally, soak your cannabis in water for 20-60 minutes before cooking.