Like any schoolmaster - the horse is much more sensitive to your weight and intention than your actual aids. This is where we must train our own seats to set an intention.
This is why it is always good practice when schooling transitions to canter to bring the outside leg back, count to 3, and then apply the aid. It makes the horse wait and allows him a count of 3 to collect up his own body in preparation for the canter depart. As well, it shows acceptance of the aids as the outside leg back is used for so many different things, travers, half pass, walk pirouettes, basic bending on circles and increased bending in voltes. The only thing which changes in these different movements is the intention of the seat and the directionality of the half halt. As well in your canter - walk transitions, you need to keep the canter aid on all the way to walk to ensure the horse does not swap leads and that he continues cantering right to the last stride and that there is then a half halt finishing the transition into walk.
Focus really on setting yourself up for success and think only about the alignment of your body and your intention in your seat. The directionality of the half halt will come - a good exercise to feel this is your basic SI and travers - but with a great focus on it simply being carrying the first and last stride of your 10m circle down the wall, respectively. That there is no change in bend and no development of the movement - it simply begins out of your circle from a good half halt.
They say money doesn't buy happiness -- well happiness doesn't buy horses!