I have contact with the horse's mouth and I sit up straight while at the same time I squeeze my hands closed (like squeezing a sponge) and firmly laying my leg on the girth (leg pressure and hand pressure are firm and a squeeze.. not a jerk or a kick). Then I release both hands and legs and continue to sit up straight. Wash, rinse, Repeat.
Pressure of the squeeze and release is determined by the horse's response. Some horses will begin to half halt just on the sitting up straight and very little pressure. Others? Need more.
A well trained horse needs very little pressure and often only one half halt to get them on the bit and collected and prepared for what you are going to ask next.