In the Academic Art of Riding, the piaffe is usually first taught to the horse in hand, starting with half-steps.
You can achieve these “half steps” by collecting the horse more and more in trot.
By halving these steps, the piaffe is then created.
When the horse shows a few proper steps during the first attempt, stop ad reward the horse extensively.
After this, the piaffe can be developed under the rider.
The rider collects the horse in trot and when the rider feels that the horse has reached its limits, the rider should ride forward again.
It will take some years before the horse is capable of perfect piaffes, but the way towards it already makes the horse stronger, more supple and better maneuverable.