“As little as possible, as much as is necessary” is more than a cliché. It is sound advice.
One underlying concept of maximum importance is that horses don’t respond to pressure. They respond to changes in pressure. I can gradually build up the amount of leg I use on my horse in a way that allows him to totally ignore it. Conversely, a gentle squeezing increase of leg which I immediately relax as the horse responds to it can make him very sensitive.
We like not to have to kick, but it is an imperfect world. Horses arrive on our doorstep with varying personalities and baggage from past training experiences. A kick is a louder squeeze. The whip comes when a kick isn’t enough. But the whip isn’t used by itself. It must not come as though the horse has been struck by a lightning bolt without knowing why or how he should react. Ask with the leg first. When you don’t get enough response, repeat the leg aid and almost instantaneously reinforce it with the whip. The whip must explain so that the next time the leg by itself has sufficient meaning.
In general, the goal is to start with a fairly small aid. If it does not draw enough reaction, go to a reinforced or stronger one and then work downward to diminish it to the least amount you need to still get a satisfactory answer.
Eventually if your horse is truly on the aids, his movement should be self sustaining as he carries himself forward, and you are able to monitor him rather than have to make him go.