To make the jump from the simple change to the flying change, the horse should be able to collect in the canter a little more since the horse will need to sit behind more. Improving the collection will also improve the engagement in the canter, which is important before asking for the flying change. 

Make sure you can collect your horse with your seat primarily, not having to pull on the reins and lose the rhythm and balance. Hopefully, you feel that this has developed since working on the reaction to your independent aids. If not, go back to Session 2 and repeat this work!

The horse should be able to collect in the canter when you use the half halt aid. Practice asking for the collection with the half halt as you will need to do this before asking for the flying change.