As alibi says, transitions, half halts have little to do with the rein It is your body that must work with his body.
Feel your body as he walks, feel it move with him. Now stop your body from moving with him, simultaneously closing your inner thigh muscles to help stabilize you. and closing your lower leg muscles against his side to keep his hind end moving up inder him. Most horses start out life listening to this, but so many have become desensitized, and no longer listen.
It is hard to do effectively at the walk but a sensitive horse will at lest slow.
You ability to do this at the trot will depend on your ability to work within the sitting trot. Many riders are happy to simply survive the sitting trot But it works the same at the trot. Your ability to control with your seat ( the amount of closing of the inner thigh, and the depth or lightens of the thrust of your seat) will control the activity of the horse’s hind end, in conjunction with your lower leg.
The half halt consists of the the fraction of the moment stoppage of your body with the simultaneously continuing forward drive of your lower leg. Should you wait til you can feel it you will end up with a halt or a downward transition.If nothing happens, rethink what and how you are attempting it.
All of this takes a tremendous amount of the human mind to micromanage all of those muscles to work simultaneously. With time and practice it is another one of those things you don’t think about, you just do.
It does help f you have someone to longe you on an educated horse. Preferably someone with patience and a good sense of humor. Because someone on the ground can talk, and explain til long after the cows came home, but is the rider who must “get” the coordination.