Cuz....You mean the Rocker Cover, yeah?? Not "rocket".
If so..remove the cover and clean both surfaces REALLY well with Prepsol or similar. Purchase a new gasket kit which should include spark plug hole gaskets and a new half moon...Apply a small thin line of rtv gasket making silicone, all the way around the head where the gasket will sit for added sealing.
Place gaskets in the rocker cover and also replace the half moon and glue that in with the silicone, then carefully replace the cover onto the fresh silicone on top of the head and tighten all the bolts starting from centre working out but don't overtighten as you will squash the gasket too much and be back where you started from.
Giddy up!!!