hold the valve up with either air through the spark plug hole, or #9 wire bent so it holds the valve up. With the piston up, even if you drop the valve you'll still be able to grab the stem and pull it right back up. I like the wire myself.
remove valve spring with over head valve spring compressor.
install new valve seal,
install valve spring/retainer/keepers
IF you want the pieces out of the oil pump screen, then obviously you'll remove the oil pan.