I prefer to mix it in the blender so it mixes better.
Put soy milk in the blender and start blending. Add cocoa powder until it looks a little darker than typical chocolate milk. Taste, and add a bit of sugar until it tastes how you like. If you aren't used to much sugar, you can get away with a lot less sugar than you'll find in typical chocolate milk / chocolate soy milk, and at the same time you can make it chocolateyer. Kind of a dark chocolate like flavor. Mmm.