Personally, my biggest requirement before I ask for the walk-canter transition is that my horse can clearly differentiate the trot aid and the canter aid. My trot aid is a hlf halt, squezze with my seat and pressure with both legs. My canter cue is half halt, Sit deep, and cue with my outside leg ONLY. This brings the outside hind under, and once they figure it out it is very easy to strike them off on the correct lead. Another big pre-requisite is MORE collection through the trot-canter transition. My horse needs to be able to calmy and smoothly take the canter, with a strong contact and moving forward into the bit. Without these two things solid, the horse will be confused about what your asking, and will try to run into the canter. For any trabnsition my horse needs to be slightly rocked back onto his hind.
For the actuall walk-canter transition, I want a very collected walk, very forward off my leg. Depending on the lead you want, there is a different time to cue. Practise feeling your horses belly swing as you walk. I was always taught that you cue the canter when their belly is swung away from the leading leg, so to the outside of your circle. I will half halt, sit realllly deep and put that outside leg on, keeping a strong inside rein.
Not sure if that will make sense, but it worked for me and my arab now has the awesomest walk-canter transitions! So much fun and easy!