Before you start, select the right type of tile, remembering that wall tiles are thinner and lighter-weight than floor tiles. Ceramic and porcelain tiles usually come with spacers, while glass tiles are arranged on a mesh backing.
Prep the surface by sanding with 80-grit paper and then wiping with a damp cloth. Also remove switch plates. Check that the countertop is level and plumb, and mark a reference line for the first course of tiles on the wall. The tops of the first row of tiles will run along this line. Also measure and mark the center of backsplash section you are tiling. Dry fit (lay out) the tiles on the countertop or the floor, and make sure the arrangement fits in that section of the backsplash. Check to see if any tiles need to be cut at the ends or around electrical outlets, or if the tile layout should be adjusted to accommodate gaps.
Using a V-notched trowel, apply the mastic to the wall. (Notches are usually 1/4" or 3/8" deep.) Spread the mastic with the flat side of the trowel, and then score it at a slight angle with the notched side. Apply the mastic only to the area you are working in—below the reference line for the first course—because it sets up quickly. Place a row of spacers along the edge of the countertop. Start installing the first tile at your center point, and then work outwards on both sides of the center tile. Press the tile firmly into the mastic (but don’t squeeze it) and use spacers between each tile. Repeat the process for the rest of the rows, working upwards and using spacers between each row as well as each tile. Periodically check that rows are level.
Once a section is complete, measure and cut the tiles for edges or corners. Hold the tile against the space and mark where it needs to be cut. Use a tile cutter for porcelain and ceramic tiles; sand the rough edges with a sanding stone. Small cuts or rounded corners can be done with tile nippers. For glass or large tiles, use a wet saw to make any cuts. Place the cut side of the tile toward the edge or corner. If the cut tile is going into an area too small for the trowel, add the mastic directly to the back of the tile and place it.
Allow the mastic to dry for 24 hours before applying unsanded grout. Prep the tile surface by wiping with a damp sponge and removing all of the spacers. Working diagonally across the tiles, apply the grout into the seams with a rubber grout float held at a slight angle. Do not grout the seam between the countertop and the first tile course. Allow the grout to dry for 30 minutes, then wipe with a damp sponge, rinsing and wringing it out as needed. Allow the grout to completely dry for 24 hours before applying a grout sealant. Buff the tiles with a clean cloth to remove any film. Run a bead of appropriate caulk along the countertop seam, and smooth with a finger.