Stir your ceramic tile adhesive. Tile adhesive and caulk are available at home improvement stores. Apply the adhesive to your countertop surface with a trowel. Spread it evenly before placing your tile on the surface.
Place your tiles onto the adhesive 1 by 1, according to your design. Place them approximately 1/2 inch (1.3 cm) apart from the next tile, to create the mosaic look. Make sure not to press them too deep into the adhesive, as the top should remain clean, with room for grout on the sides.
Continue adding tiles to your mosaic until all of the pieces are placed on the countertop. Allow the tiles to dry for 24 hours, or as long as the adhesive directions indicate. Mix your sanded grout according to package directions, or buy a pre-mixed grout.
Apply the sanded grout with a sponge squeegee. Do small areas of the countertop 1 at a time, so that you can be sure you have filled the spaces completely with grout. Move the squeegee diagonally across the surface before moving vertically or horizontally.
Mix a weak solution of white vinegar and warm water to clean your tiles while the grout is still wet. Dip a sponge into the solution and wring out the excess liquid. Wipe your tiles until they appear clean.
Leave your grout to dry according to package instructions. Apply 1 to 2 coats of water-resistant sealer to your bathroom countertop with a paint brush or sponge. This will help to avoid mold growth and make the counter easier to clean. Allow the sealer to dry well between coats.