On my Vitesse I made sure the Head surface and recess on the Cover were clean and flat, I used a light continuous smear of Blue Hylomar to stick the gasket to the Rocker cover recess, then carefully bolted it in place.
I used a small rubber 'O' Ring the same diameter as the stud, then a fibre washer, then a flat washer under the nyloc nuts to stop leaks
Also don't overtighten the nuts or you will distort the rocker cover.
Worked for me
Regards
Gary