Drive a blade into the silicon to start prying it up. When you get it open carefully scape every bit of silicone off before reinstalling the valve covers.

Exacto knife - scrap it off with the flat .5" blade.

Use scraper n after scrub it with rags with aceton or paint thinner to clean thoroughly..

Use a sharp, flat blade like a putty knife to scrape off as much as you can of the old gasket and glue. Be careful not to damage the valve cover (or cut yourself) while you're doing this. Once you get MOST of it off, you should be able to use a silicon glue solvent to remove the rest. (look for it at your local hardware store)

The valve cover mating flange MUST be absolutely smooth and clean before you install the new gaskets.

this addresses removing the material once parts are seperated. to remove a stuborn valve/rocker/cam cover, use a block of wood on the side of the cover. hit the wood with a hammer. wood protects the cover from the hammer blows. should knock it free to the point you can now review the clip.