With the plugs all out, rotate the engine to tdc on cylinder your replacing valve stem seals on....piston will stop valve from falling into bore.
or
Feed rope into bore with motor at bdc...rotate to tdc or untill rope squeezes
against head.
or
Use air pressure...make sure you finish that bore/valve before air runs out.
I use a shop air hose at 185 psi....with piston at bdc.
Never lost a valve yet
You will need to rap the keeper retainer to break loose the collets.
Use a socket and engineers hammer .
Compress valve spring..
Use a magnet to remove collets. Remove retainer/spring.
Pull off seal...renew seal...replace spring etc and compress to insert collets.
WARNING.....
Put clean shop clothes in any drain holes and other places that suck up small flying pieces of engine.
Do not do this job if there is any chance of interuption...ie..thunder storm...tornado...loosing air pressure...etc.
.