First of all think, "walk on briskly!" or "Trot!" . You'd be surprised at how well your INTENTION comes through when it's really IN your mind.
For trot, I would put both legs on, and bring up internal energy in my body and think "trot" . I hope that doesn't sound too "woo-woo".
For Walk; you put on one leg, alternating with the step of the horse. You will want to put your right leg on just as the horse is lifting HIS right rear leg forward and viceversa. How will you know? Well, let your horse walk around a bit and really feel the horse's barrel roll from side to side as it walks. When the barrel rolls to the left, that is becasue the right rear foot is stepping up and under that fat belly. That's the time to cue THAT side. So, to amplify a walk, you rythmically cue the horse, WITH his stepping under.
Remember, when you get the amplified walk, stop cueing and let the horse be responsible to carry that. If you constantly cue, you will dull him out.