What I do with mine is on the ground to start with. When I start leading them I tell them "walk", and give them a light tug on the lead rope. Give them about 3 seconds to respond. Then I tell them "walk" and kiss, then give a light tug on the rope. Give them another 3 seconds to respond. If it doesn't work I tell them "walk", kiss, and put lots of pressure on the rope. For the most part it doesn't take a lot for them to catch on. The when I transition to riding, I do, for the most part, the same. Tell them "walk", give them 3 seconds to respond. At first even one step is good. If they don't respond, I say "walk" (always in a firm voice that leaves no doubt what I mean), and kiss. Give them 3 seconds to respond. At any point if they do respond correctly they get praised. If they don't respond I move on to saying "walk", then kissing and then bump with both legs. From there, if they don't respond I repeat the "walk" "kiss" and leg bumps with more and more intensity each time. Smart one's will pick up on the fact that they get two tries before they get leg and usually move off pretty quick.
Same goes for the trot only I cluck three times really quick instead of kissing, and when I kick, I tap twice real quick. Later they learn that one tap (bump, what ever you want to call it) means walk and two quick taps means trot with no voice cue to go with it. When I move to the lope/canter, it's still the same process. I use the word "canter" and a kiss and leg. But with the canter I use the word and/or the kiss combined with the leg. That way the learn which lead to take. When asking for the canter I try to start them in a smallish arena that way I can use the arena to help them learn leads. Typically, they will take the correct lead in a smaller area pretty quick on their own cause it's more difficult for them to "counter-canter" then it is to canter on the correct lead. Again, I tell them "canter" and bump with the heel to tell them which lead. If they don't get it right away then I tell them "canter", kiss, and bump a little harder with the heel. Eventually, All I have to do is say the word "canter" and bump with a heel OR kiss and bump with a heel and they go into the canter on the correct lead.
The key difference between the walk "kiss" and the canter "kiss" is the heel.