Sand the surface of the mantle using an electric sander with a medium-grit sanding belt.
Vacuum the mantle and surrounding areas using a shop vacuum to remove sawdust and debris.
Spread a 1-inch thick layer of medium grout over the entire surface of the mantle using a trowel. Apply the grout in an even layer.
Press 1/2-inch thick stone tiles halfway into the grout, spacing each stone 1/4 inch apart.
Scrape the trowel across the stones to push excess grout into areas where grout is needed. Wipe off the surface of each stone with a damp washcloth. Allow the stone mantle to air dry for 12 to 18 hours.
Apply a thick layer of stone sealant over the entire mantle using a 2-inch flat paintbrush. Allow the stone mantle to air dry for 12 hours.