Cut a a solid piece of wood using a circular saw to measure 12 inches wide, 6 inches high and 60 inches long. Select any type of wood that you prefer.
Sand the entire surface of the wood using an electric sander with a medium-grit sanding belt. This will eliminate splintering, roughness or areas of unevenness from the wood mantle.
Wipe off the mantle with a washcloth to remove any sawdust or debris.
Soak a sponge brush with wood stain, then apply the wood stain over the entire surface of the wood mantle in an even layer. Allow the wood stain to dry for six hours.
Apply a second layer of wood stain if preferred, then allow the stain to dry for an additional six hours.
Coat a 3-inch flat paintbrush in clear polyurethane sealant, then apply the polyurethane over the entire surface of the mantle in an even layer. Allow the polyurethane to dry for eight to 10 hours.
Apply a second layer of polyurethane sealant, as before, then allow the mantle to dry for an additional eight to 10 hours.