If you want to lap the head, typically the spigot will be much too high to do this by just lapping alone.
A good method would be to use a lathe to bring down the height of the spigot to be just a hair longer than the depth of the recess in the head.
You can check it by putting the head on the barrel and pushing it down all the way to be sure it's fully seated home, without any head gasket in place. Measure with an automotive feeler gauge, the empty gap where the head gasket would normally be, and that is your gap which needs to be narrowed. Reducing the height of the spigot carefully bring the gasket mating surfaces very close together so that very little lapping will be necessary, is the goal. Frequent checking of the gap by putting the head on the barrel will help you to gauge your progress, so that you don't shorten the spigot height too far.
The goal is to have the top of the spigot mate perfectly home into the head recess, at the same time that the mating surfaces on the head and barrel(where the head gasket used to be) come fully home together.
This will be accomplished in the final stage by lapping with valve grinding compound, as Royce Creasey describes.
I have found that lapping often allows some oil seepage to still occur. It's very hard to eliminate it without some sealer. So, a very light smear of hi-temp silicone sealer around the pushrod tube area should take care of any slight imperfections, and seal the oil.
I generally do not do the lapping process, but on occasion we have done it for some people who are at high elevations that need to increase their compression to the maximum extent possible because of thin air conditions at their locations in the high mountains.