didn't mention whether you ar re using the old gasket, or installing new...
so,
if re using, carefully peel it out, clean and scrape the cover groove wtih a screwdriver, and wipe clean with non oily solvent, (MEK, Acetone, Denatured alcohol, BrakeKleen, or even GumOut..which is my standby.)
clean the head surface also, in a similar manner, using a single edged razorblade if needed... to remove all the other Horse Hockey Silicone someone added before....
Then, clean the actual gasket, gently but completly, with a rag soaked with one of the aforementioned solvents..
I don't really condone adding any silicone to the gasket/valve cover interface whatsoever, as anything more than a 1/16" bead, tends to migrate into the inside, and end up blocking oil passages, and internal oil drains and filtration... seen a lot of this, and people just goober that crap, the red stuff,on the cover/seal interface, and also the seal/head interface, willy nilly.. (seems like EVERY valve cover I pull off, I have to spend 15 minutes cleaning and scraping that crap off, and picking bits out of the area below, really pizz's me off) and if I do use anything, Ultra Black is what i use, but only on really old over compressed gaskets there.. and a miniscule bead as noted.
If anything, I usually spray the gasket surface that contacts the cover, with a spray 'contact cement' (or even a very light finger smeer of the stuff in the jar) , and invert the cover and gasket and place a couple phone books on it till dry... then it sticks.. The contact cement dries fast, and re creates what the original factory Kawabond glue they used from the factory...
the ONLY place where I use sealant, is in the 'crescent' cutouts at the head, and a VERY LIGHT finger schmeer of the Ultra Black RTV Sealant is used, ( not a BEAD, a light schmear..mmkay?) just moments before dropping the cover in place..
if you see sealant OUTSIDE, smooshed out, might as well figure 3x as much may have gone INSIDE...
if using a NEW gasket, I suggest the contact cement, and Black RTV (on the crescents again, ONLY) and install...
the rubber gaskets on C10's are very robust, and many last the life of the bike, if not abused, and continually slathered with silicone goop, which makes it almost impossible to adhere anything to down the road... and frankly, it's a really unnecessary P.I.T.A. to have to clean up, every time a cover is pulled.