The valve cover gasket on my 1998 Ford Escort zx2 has failed. I have ordered a new gasket and plan on replacing it myself because it looks easy enough. A few videos I watched indicated that a sealant should be used, but they didn't mention specifics. I already have some red high temperature RTV. Can this be used? Should it go on both sides of the gasket or just one side or the other? Should it go over the whole perimeter or just the corners?
Provided that the red RTV you have is oil resistant (which, according to the packaging you linked, it is), it should be fine for this application. The packaging specifically says it's intended for valve cover gaskets.
In my experience, you normally apply RTV to seams in the sealing surface and at sharp corners, but otherwise should keep the RTV to a minimum. If nothing else, applying it unnecessarily makes a mess and makes it harder to take it back apart without destroying the gasket.
Be sure to follow the torque and application requirements of the RTV - it's not always just to apply and torque down! Some of them have waiting and curing times before torque should be applied.
RTV sealant should never be used on gaskets. RTV sealant will cause the gasket to slide out of position resulting in a leak.
On cork valve cover gaskets, RTV sealant is only used to hold the gasket in position while placing the cover on. A small dab (just enough to hold the gasket in position) is placed in each corner.
There are instances where RTV is recommended on gasket surfaces. Such examples include where two case halves come together to create a gasket surface, or where the gasket makes a sharp bend in a different direction.
In your car, the gasket should be a rubber gasket that sits in a groove that holds the gasket in position. So there is no need for RTV.
Simply remove the valve cover and pull the old gasket out of the groove inside the valve cover. line the new gasket up in the correct orientation and press the new gasket in the groove with your thumb. Reinstall the valve cover.