I glue the gasket on one side into the valve cover and then let it sit flat on my work table for a day or two. In mid-winter, I have lots of time. Then I "adjust" the gasket a bit where needed so it sits evenly all around and I let it sit for a few more days. When I lift it off the table to install it, I take solvent and clean any excess goo off the face of the gasket that will mate with the head.
This is as Randall describes. But I don't use any sealant or grease on the mating face. This way, the gasket is good for 10 years or more, even with periodic checks for valve clearances, etc.