Right, first thing to do is try and figure out what problem was being addressed with the double gaskets; Rocker clearance from non-original rocker gear? Valve covers with deformed flanges or oversized holes, etc. If rocker clearance is ok with a single gasket go with a single new cork gasket, clean the flanges of the covers and the rails of the heads with a good solvent like acetone to get them clean and dry. I never use gasket sealer of any kind on either surface.

Trial fit the gaskets to get them seated well into the valve covers, place them on the heads and make sure the flanges and gaskets sit flat everywhere, the gaskets need to fit and follow the flanges, takes a bit of finesse sometimes. Tighten the bolts to just "snug" don't overtighten them! then follow around and snug them again, I generally grab the head of the ratchet and twist with my wrist rather than to go out on the handle adding a lot of torque. Once you fire and warm the engine go back and re-tighten all the nuts as the gaskets often get soft and leak after the first fire up. .02