Thank you for your question!
If the horse already knows the piaffe then the easiest way is usually to ask them to gently move forward from the piaffe into the passage.
A few things are important:
1) you should have the feeling that the horse does the piaffe "by himself". So you shouldn't need to push, kick, whip, scream or cry to have your piaffe with enough energy and regularity.
2) when you will ask your horse to start moving forward, it should be done only with your seat. The feeling is almost like having your own body moving 10 cm forward (sorry for the centimeters, I let you Google it if you need the inches!) before the horse comes with you. It is not about leaning forward, you should really have that feeling that your whole body goes forward. You want to be in the passage before the horse, it will invite him to follow you without loosing his collection or the rhythm of the piaffe. But be very invisible with that, you just want your horse to feel the invitation, not the make these ugly movements with your belly!
3) Do not look for expression in the beginning. Of course we dream about these huge, super expressive passages, but in the beginning you want the horse to understand the idea, the suspension. Expression comes last. What you need first is that the horse doesn't loose the rhythm and the mental and physical relaxation.
4) Don't move left and right with you seat. It is ugly and very inefficient. But you will probably do it anyway, because we all do that at some point...
5) Think that it is like an extend trot but what the horse will loose in his forward motion, he will use it for suspension. What he looses forward, he uses upward. What he looses in speed, he uses in suspension.
6) If the horse doesn't find the rhythm when you ask to passage coming from the piaffe, try for a while to keep a little bit more contact with your legs during the piaffe, and keep that steady pressure when you want to enter in the passage. With some horses, it makes miracles. It helps them to keep the rhythm but also to stay round and to understand how to jump and have suspension. Of course, that doesn't mean to press him like a citrus, just to have a little bit more contact than usual already during the piaffe, 5 or 6 steps before entering in the passage.
I really hope this will help!