I'm making new cushions here, but you can gently pull off the seat from any chair to make it poofier (don't you love that word?!).
To begin you'll need some fabric cut in the shape of your chair seat. I'm using burgundy velvet here. You'll need half of a jumbo cotton ball, and some heavy cardboard for the base of the seat (if you're covering a new chair - you can just cover over your existing chair seat). Scissors and a glue gun.