Improvise along with a backing track
Listen out for a little tune you really like when you play, try to repeat this tune lots while you improvise.
Then try to write your tune down.
You don't have to use musical notation on a staff, you can come up with your own way of writing the music down, just as long as you know what it means to be able to play the music again another day.
Try to work either the NOTES first or the RHYTHM first.
NOTES - look at the notes you were playing on the piano and then write them down.
RHYTHM - try to work out whether it is a ta , or a titi and so on. If your rhythm is quite complicated then just focus on the notes.
Try to remember a KEY SIGNATURE if you need one