The first thing you need to do is dink the keepers loose.
Remove the rockers and then take a 1/2" socket and place it on top of the valve spring and centered over the stem/keeper. Then give it a smack with a hammer. It will jar the retainer from the keepers. It won't come apart but will loosen it up.
THEN, bring the piston to TDC for that cylinder and apply about 30 pounds of air into the cylinder. Now you can depress the retainer/spring and remove the keepers easliy.
I use the old flat spring compressor lever, but then again, I don't do it very often. It looks like a flat crowbar and bolts to the rocker stud and you just lift it up and it depresses the retainer.