I like using a gasket tack to adhere gasket to valve cover, let cure, using light clamps and straight edges, to keep gasket formed to shape of cover. then use oil resistant rtv on gasket to head, with good results.
I use the same method, but I use RTV on both sides. Plus I dab a bit on each bolt to keep any oil from seeping UP the valve cover bolts and staining the lip of the valve cover.