Bring piston to TDC on that hole, replace spark plug with air fitting, apply air pressure to keep valves up, pop out the keepers on the spring retainer, remove retainer & spring, install seal.
With a shaft mount system it's probably easier to remove the rocker shafts; loosen bolts in stages to avoid the shafts getting bent. On my Street Hemi it was easy to just slide the rocker arms sideway on their positioning springs.
The keepers are small enough that they can easily fall into the heads, so use a lot of shop rags - preferably lint-free - to catch them.