When you ask your horse to go to trot give one big kick, and then like Caoimhe said, don’t touch your horse because it will teach them to ignore your leg. Don’t touch your horse with your leg until you feel them begin to stop or slow down, and then give them another kick.
I hope this helps answer your question!
Doing little kicks is a very bad idea. That's exactly how you teach your horse to ignore your leg completely.
This is one of those situations where the use of a crop is appropriate. If the horse ignores your first kick, follow it up with a kick and slap (in saying that, I prefer to use a voice command first). One big kick/slap with a stick is much more effective than lots of little kicks.