Loosen and remove the light switch wall plate retaining screws. Remove the wall plate from the light switch.
Place the gasket over the light switch. Press the gasket into the light switch box so that it fits snugly over the light switch.
Hold the gasket in place and replace the light switch wall plate. Insert the screws through the light switch wall plate and through the gasket holes. Tighten the screws so that the wall plate holds the gasket snugly into the switch box.