First lift your hands like 2-3 inches and spread them apart 1-2 more inches than normal, and in your mind just think "up hill"
Put your outside leg back acouple inches, just behind the girth and squeeze.
And just while your getting better at it ask in a corner of the arena, the turn will also help the horse pick up the lead.
Make Sure you sit the trot first, then outside leg back and put the inside leg on and the horse should canter. But make sure you sit the trot first other wise the horse will just think you want an extended trot.
To ask for canter you put your outside leg back and squeeze and continue to squeeze with your legs until the horse canters. If the horse doesn't canter from this cue, there could be a variety of reasons why, including;
- The horse is green and doesn't yet understand the cue/has learnt to ignore the leg from too much leg pressure
- You don't have contact with your leg and your horse
- Incorrect riding footwear. I find long boots are much easier to communicate with (contact wise) with the horse than ankle boots.
- The horse's respect for you is minimal or non-existent
- You're unbalanced in the seat so your horse continues to trot
I'd look into these problems and see if any apply to you and your horse. Have hard work and patience and you'll be transitioning to canter like a pro!