Well, I never did it on a G, but I have done it on several other cars. So my advice may not be much help, but better than nothing.
You can do it with the heads still on the engine block. The trick is to keep the valves closed. You do this by removing the spark plugs and inserting an adapter in the head where the plug went. You can use the adapter from a compression tester.
You rotate the crank to TDC compression stroke of the cylinder you are working on, so both valves are closed. You then put shop air (90 PSI) in there. If it is a manual, make sure it is in neutral! After you remover the valve covers, you use a valve-spring compressor (lever-type, not C-clamp) to compress the valve spring, remover the 2-piece keeper, pull the seal.
Reassembly is the reverse.
That is the basic process.