The first thing you need to do is to write down what the music reminds you of. Is it a scampering insect, a floating leaf, a noisy train station or a barking dog? There is no right answer here. Write down what ever comes into your head.
It may be that different sections of your piece remind you of different things. If so write those down as well.
The next step is to use your music to join those ideas up into a story. For example things like notes going up can be climbing the stairs. Long notes could be having a sleep. Big intervals can be jumping.
To finish write your story in pencil above the notes on your music. Then when you play the music think of your story.
This is a lot of fun, can be quite silly but will definately improve the shape and expression of your music.