It's pretty simple if you know anything about cars I guess...
I don't know if your local parts store like autozone have the gasket or not, but they're usually pretty cheap, no more than 100 dollars. Buy some high temperature rubber sealant for the gasket. Remove all the things that will get in the way of removing the valve covers. Remove the valve cover. Remove the old gasket, you might need to scrap it off. make sure nothing falls in. Use the rubber sealant and and apply it on the the block, then place the gasket on, finally apply more rubber sealant on top of the gasket and bolt back on the valve cover. Put everything back together. Wait 24 hours before driving.
To be sure you have a leak. Sometimes when you change your oil you might have dripped from recent or old oil changes. Spray the leaking part down with some break cleaner, and if you see a definite leak, THEN change the gasket.