Next take your music where you want it to go and write a first draft. Perhaps it helps to think of a picture your music describes. Maybe your piece is built around a strong emotion. Or maybe you want to somehow convey spiritual or moral ideas through your music. For your work to develop, your themes must devlop.
Themes are like characters in a play.
Although you must find your own ways to solve the problem of how to develop, you can follow some common practices. You can flip the theme around, invert it, play it backwards, transpose to other keys, alter harmony and rhythm, add notes, subtract notes, lengthen the values of each note, cut pieces of the theme out be creative with your development. The only rule is it has to make sense. When you reach the end, the audience should know it's the end. Make sure your conclusion is direct.