Often we improvise on the organ using a hymn tune or a chorale as a thematic material. This is especially useful in a church setting.
Then we can work with various forms, such as chorale prelude, chorale variations, chorale ricercar or chorale fantasia.
But what about a free piece based on a theme which you come up with yourself or borrow from other composers?
For this to happen you need to know things like how to create an exposition, ABA form or a verset.
If you're interested in learning to create them yourself, check out my video training "Improvising Without a Chorale" (50% discount is valid until December 14).