Do one cylinder at a time and bring that piston up to the firing position. The valves won't fall into the cylinder that way. It's the same procedure when adjusting your valves. Mark your damper at top dead center for #1 and then at 90, 180 and 270 degrees. Start at cylinder 1 follow the firing order turning the damper to next mark before starting that cylinder.
1: Get whatever piston you want to replace the seal on up to TDC with the valves closed.
2: Tap the top of the spring retainer with a rubber mallet to get everything unstuck. If one of the spring assemblies pops apart at this stage you will have to hold the valve up by hand until the next step. This is the reason you have the piston at TDC, so the valve wouldn't drop into the cylinder.
3: Rotate the piston down to BDC with the valves closed.
4: Attach your air adapter and pressureize the cylinder.
5: Use a tool like this one http://store.summitracing.com/partdetail.asp?part=PRO-66848&autoview=sku to remove the spring, retainer and keeper.
6: Replace the seal.
7: Put everything back together and do the next cylinder.