Let's start with the less-expert: I teach the following steps.
1) Bear down* and check your spinal alignment.
2) Make sure you have a light contact with the horse.
3) Squeeze the reins like you're squeezing the water out of a sponge.
4) Keep bearing down and slowly increase pressure until the horse halts.
Erika Prockl: Practice first quarter-halts, then half-halts, before attempting a full-halt. It's important to keep your feet relaxed (don't push down with your stirrups in order to "grow tall"). She refers to blocking the impulsion with your hands, but it remains unclear what she means by that.
George Morris: "To use the hands, one closes on the reins as if squeezing a lemon. In basic, slow work, it is wrong to pull back on the reins." He encourages teaching the riders to back up first, as a way to establish coordinated versus clashing aids. He then discusses getting a horse on the bit...
Anna White-Mullen: "A good transition is smooth, with the horse's hocks engaged." and later, "Transitions are important at all levels of riding, for they demonstrate the degree of the horse's obedience and, consequently, are a means of assessing the rider's safety." On the quick horse, she adds, "half-halts reprimand the quick horse, but halting is even more effective because it requires the horse not only to slow down, but to obey to the point of immobility."
Mark Russell: In-hand, "In an educated halt, the hind legs come forward and stop in balance under the horse. Pulling back on the rein actually interferes with the horse's effort to balance himself and is apt to create tension. Instead, closing the hand and fixing the rein on the withers allows the horse to step underneath his barrel; as he steps into the rein, he stops forward movement and halts. Fixing the rein, once the horse understands how to release to it, helps the horse maintain his balance."
Andrew McLean: Have a contact first, then brace your back (by keeping your shoulders above your hips.) Push hips toward stirrup bars. Don't pull back, but keep your elbows to your ribcage and close fingers to make a fist. Hold until desired response is achieved. "The rider should imagine making the lower back broader and positioning his pelvis straight downwards." Close the thighs to stabilize lower body. At the canter, the correct time to apply downward transition aids is when the hand moves backwards.
Jean-Claude Racinet: "A careful adjustment of the reins is an absolute prerequisite." While properly adjusting the reins, your arms might move about a bit but "as they act, the hands should never come closer to your body)... The hand must remain immobile. The action is provided either by the fingers (clenching), or by the torso (reclining), or by both." JC then says that a common error is to recline the torso but give the hands forward while doing so, essentially not saying anything to the horse. He also says that "a slowing down is nothing else than an uncompleted halt.... if we call the action of the torso by which we have stopped the horse going a slow walk a 'unit of stopping', slowing down a horse from a high speed will require several of these 'units.'" JC is adamant that all aids are pulsed for their required result, "the action should not exceed one-half second."
Anne Kursinski: "To come back to the halt, first stop any driving aid you may have been using with your legs. Then increase the feel on the reins the same amount you increased leg pressure at the walk (just closing your fingers more firmly should apply sufficient backward contact). As your horse slows and begins to stop, decrease your hand pressure to tell him he's doing the right thing. Repeat the increasing and lightening until he halts (you should need just a couple of squeeze-releases for this simple transition)." Anne explains the importance of not allowing the horse to change your position as you halt, as well as deepening the seat. I particularly like this image in regards to not getting an uneven contact: "the feel of elastic contact is the same as the one you need to pick up and set down a full bucket of water without spilling a drop. If you jerk the bucket, you'll spill the water; if you're limp, you'll never lift it at all."
Franz Mairinger: Education of the horse is making him "go as quickly or as slowly as you want him to." The outside rein must check the horse from the canter, and the inside leg keeps him going. Be subtle, and do not allow the horse to speed off at the trot. Do not allow the horse to change his flexion. "How do you go back to a walk from your trot? All you have to do is sit down with your weight in the saddle and stop the forward motion of your hands. Give the horse your weight and he will walk. You must have contact or else he will run away. If that is not enough at first, use both hands into your seat. But sit down first, then use your hands." Then at the end of the chapter there's this delightful summary:
"To check from canter: Outside rein increases pressure but not to pull his head out or change his flexion. Inside leg takes over and holds him forward into the trot. The first steps after a transition are most important. He must go smoothly into a true trot, and should not shorten the stride of his canter.
From trot to talk: Put the weight down into the saddle, and stretch a little more. Stop the forward movement of the hands. Have contact but do not pull."
Severyn R. Kulesza: "To decrease the pace or to stop the horse, the rider acts with his hands. He pulls the reins in a straight line towards his elbows as strongly as is necessary. As soon as the horse obeys the order, the rider stops pulling and immediately returns to the previous passive contact. Never start pulling with your fingers, because you stiffen the tendons and muscles, and the horse feels hard pressure in his mouth... then the horse will feel soft pressure in his mouth and will not resist." He does make an interesting comment about when the horse is resisting us: "(when the horse resists) we them stiff, but only of one hand. The other stays soft and relaxed. Then the horse cannot pull our hands. He meets an unpleasant hard rein on one side of his mouth and punishes himself. In other words: when the horse resists--we resist also, but only with one hand. This is the only moment in riding when the rider makes his hand stiff. As soon as the horse's resistance vanishes, the hand immediately returns to its passive contact."
Major Anders Lindgren: "Never get stuck in the horse's mouth." Step 1) Squeeze the rein, and positively reward every slowing by a yielding of the rein. Be prepared to repeat. Step 2) More regulating influence is given when the rider repeats step 1 and angles his wrist, turning the palms to the sky and moving the pinkie towards the bellybutton. Step 3) "If the horse still resists: Repeat steps 1 and 2, and now - if necessary - the rider for the first time uses his elbow and shoulder joint."
Paul D. Cronin: Cronin recommends a check and release program for the horse first learning about the rein aids. His first phase of training allows the horse total freedom of the head, with contact only being taken for turns and halting. The check and release should be soft but increasing in intensity until the horse has responded.
Reiner Klimke: "To begin with we should let the horse take its time before coming to a halt. Slowly we improve on this until we can halt at a prescribed point." The aids for a halt and half-halt are the same, but it is unclear what those aids are. He describes it being necessary to have a "pliable wrist" and employ the use of the restraining weight aids.
And there we have it! I've skipped a great many books in my library for fear of not finding a definitive answer or because they're primarily showjumping books and Tango needs more dressage skills (yes, there are some pretty awesome show jumpers up there, but Anne Kursinski is notorious for her love of dressage), and all of my Mary Wanless books are currently lent out, which is why we don't see her here. I've also lent out my book by Thomas Ritter, and countless other dressage-y books that might help. Alas! So let's put my 'friends'' advice into practice.
*Bear down. To teach an easy way to activate deep core muscles, Wanless had a rider clear her throat so she could feel the push against her abdominal muscles and the tone in her back muscles. Bearing down enabled her to match the forces the horse's movement exerts, giving her more stability, control and better half halts. - See more at: http://dressagetoday.com/article/rider_biomechanics_081809#sthash.Y9LX889O.dpuf