post "higher" and add leg while still keeping a grip on the horses face keeping it from breaking from the trot to say a canter. by higher i mean lighter on the sit and more stand.... don't over do it, you shouldn't be able to see it if simone were to take a video
First, you want your horse to have a great medium trot with impulsion and his hindlegs engaged.
If you are not used to ask extended tro, I advise you to do this exercise : for example do a 10 meter circle in S and then do the S and P diagonal at extended trot. The circle allows you to collect your horse before asking to extend.
The first times you do it, ask him gently, you do want your horse to speed to speed up his trot without extension.
I would also advise you to work it with someone to watch because the first times it's hard to feel the difference between a horse who speed up and a horse who really have an extended trot.