Music Composition Watch
To help think about what type of audience you've written the piece to; my free composition was toyshop and I made it specific for children, so my piece was light and playful (for example)
You could write about what timbres you've used and what mood it brings
Also think about harmony, what chords you used. Also mention the melody if you do so.
There is a lot you can mention. I did AQA, so it gave us a list and we chose 3 to write