Both of the above answers are what most do but nowhere do these aids allow for the different movement of the leg from the walk to the trot to the canter.
Both have said "squeeze" but if you referred to the half halt thread any squeeze at any time will affect ONE set of legs and not all four. This means that one hind leg will get the message...but not the other ( resulting in possible crookedness).
Also I would be asking for an aggressive ( under control) movement from one gait to the other.
Three gaits...should equal tree slightly different aids.