Giget is correct if you want to have the horse back on the ground.
If you want to it to back while you're riding, first, bring your pony to a square halt. (just standing nicely, focused but relaxed). Keeping even amounts of pressure on both reins, bring your arms back towards your body; don't raise them up towards your head as you do this. Apply even amounts of leg to hand pressure, this way the pony knows you are asking for movement, not just yanking on his mouth.
Slowly increase the leg to hand pressure, and encourage with little clucks if neccesary to get the backwards movement. Only do a few steps at first, maybe three. Then release the pressure and reward with pats, etc.
Hope this helps :)