I am again learning about music composition by composing. When I sat down with my guitar tonight I was indecisive. Should I work on older pieces, or investigate the idea that began last night. Even bring "White" back from last weekend back to look at. I choose to explore the new compositional idea.
There are exercises in books on composition, and I am certain I could learn that way also. But time is precious and I am a guitarist, so I practice my composing by working on solo pieces for the guitar. Really quite simple and really quite powerful.
To paraphrase & modify the words of Robert Henri - If you are studying music and not making compositions, my advice is to begin immediately. Back to work.