Lay thin-set mortar on the surface of the countertop with a notched trowel. Apply a thin layer, about 1/2-inch thick. Smooth out the surface of the mortar with the smooth side of the trowel. Begin at the front edge of the countertop and work in small 2-foot sections to avoid having the thin-set dry before setting the tiles in place.
Set tiles onto the thin-set. Start near the edge of the countertop and work your way back. Set tile spacers on the sides of the tiles after setting them in place.
Apply more mortar to the surface of the countertop, as needed.
Measure the spaces where full tiles won't fit with a measuring tape. Transfer the measurement to the tile, using a fine-tip marker to mark the measurement. Cut the tile with a wet tile saw over the marker.
Remove the spacers from between the tiles. Allow the tile to set for 12 to 24 hours once all off the tiles have been laid.
Apply waterproof grout over the surface of the tile with a grout floater. Work the grout into the spaces between the tiles. Wipe off the tile floater and run it over the surface of the tile to remove excess grout.
Wipe down the surface of the tile with a wet sponge before the grout hardens to remove and clean the grout from the surface of the tile.