The piaffe is, in a sense, the epitome of collection at the trot. A pony performing the piaffe is highly collected and is trotting in place. The piaffe is an elevated trot where the pony appears light, but has no, or very little, forward movement. The piaffe gives the illusion that a pony is nearly weightless, while, at the same time, appearing very powerful.
In principle, it is relatively easy for a human pony to perform the piaffe since it is essentially an in-place elevated trot. However, in practice, achieving the apparent lightness of movement is actually nearly as difficult for a human as it is for a bio-horse. Of course, a bio-horse will usually have the added weight of a rider while most human ponies performing the piaffe will be show or dressage ponies and will likely be rider-less. Nevertheless, most human ponies will find performing the piaffe surprisingly exerting.
A ponygirl or ponyboy can achieve a basic representation of the piaffe by simply trotting in place. This actually produces a quite reasonable emulation of the movement. Trotting in place is typically enough to get the point across to a trainer or judge of what movement is being performed.
However, if you wish to build upon the above to better reproduce this advanced movement, you can teach your pony to incorporate a higher step into this in-place trot. The higher step should still maintain the rhythm of a trot and not be jerky or stilted (it is more important to maintain an even rhythm than to step really high). Gradually have your pony lift its legs a little higher and keep them in the air a little longer until you have a very light appearing trot.
This will likely be the hardest part for the pony to master. Even the most fit of ponies may have a hard time high stepping in place at a frequency and cadence consistent with a trot, while still maintaining the appearance of lightness consistent with the piaffe. Thus, do not be frustrated if this movement is not easily mastered by your pony.