How to write a Cadenza

I'm playing the Quantz concerto in G for an audition, and my teacher wants me to write my own cadenza for the solo section. I've tried but nothing sounds right, anyone have any tips for this?
Thanks

Choose one of the themes from the movement. Play it in its original key and form, and then try to start varying it and see where it takes you. Don't worry about tempo or strict meter. Be free. But do try to stay in the style of the period!