I usually just start doing something. Side projects don't need to be planned at all. They are just stuff we do. For example, one of my favorite type of side projects is fixing stuff. I have an in-human ability to do so. To give you an idea, today I changed the timing belt on the wife's car, and fixed a neighbor's washing machine (a sock got stuck in the pump). I'm also about to start writing code on a little API im writing for fun. Just start doing something without planning so much. It's easier to decide along the way than to figure out everything from the start.
