1. Using 1/4-inch-thick plywood (or plywood the same thickness as the mirror), cut a decorative pediment that extends above the light fixture. Cut an opening for the fixture. Nail the board to the wall. Caulk the seam between the top of the mirror and the plywood and around the light fixture. Paint the plywood silver.
2. Begin gluing shells to the top of the pediment, using a large featured shell in the center. To secure heavier shells, hang them over tiny nails as well as gluing them. Glue shells along the top curved edge until the pediment is covered.
3. Remove the 1-inch-square tiles from the paper backing by soaking them in water. Glue the tiles to the mirror using clear acrylic adhesive caulk. Butt the tiles together to eliminate the need for grout. Attach a border of four rows of tiles along the bottom of the mirror, then build up the sides with rows of four tiles, working all the way up to the pediment. Trim tiles as necessary to fit around the light fixture and the edges of the shells.