Do you want the long or short version??!!!
Half halting is something that develops over time, much like collection.
You start by teaching basic transitions. Until you can make a downwards transition as soon as you THINK downward, then you won't be able to ride a half halt.
When you transitions are on the spot, then start riding towards a downwards transition, and just before the horse changes gait, ride forward again.
This gradually becomes so refined that there is a nearly imperceptible shift of balance to the hind legs, with seemingly no aid from the rider.
A true half halt is ridden through only the seat, with no rein required, and is intended to rebalance the weight over the hind legs, and to activate the hind legs while bringing the wither up.
It is something that takes years to develop, there are whole books written on the subject of half halts, with much debate and ideas on how to ride a half halt - whether to ride it with the lower seat, to half halt through the base of your neck, to sit deeper or to lighten the seat... etc.
Its complex ;)