Before you prepare the mortar, clean the substrate of all dirt and oils. Place the tiles where they will go, with plastic spacers for the grout lines, and make adjustments as needed. Aim for a symmetrical look, with no narrow slivers of cut tiles.

If possible, position all cut edges at the back of the countertop, where the backsplash tiles will cover them. Using a wax pencil, mark cutting lines on the tiles.

For a countertop that turns a corner, start the layout at the inside corner. If the layout ends with a very narrow sliver, slightly widening the grout lines may solve the problem. In cutting the tiles, take into account the width of the grout lines on either side.

A tile cutter does an excellent job of scoring and then snapping tiles to fit your layout. If the tile you have chosen is particularly hard, you may need to cut it with a wet saw, available for rent at most home improvement centers. For fine-tuning, tile nippers can break off tiny pieces to create curved or complicated profiles.

1. If you're using edging tiles, mark a line along the edge of the countertop to allow for the tiles plus a grout joint. Then place a guide strip along this line and temporarily attach it to the countertop with nails. Lay out the field tiles from the edge of the strip to the back of the countertop, using a straightedge to align them.
2. Mix the thinset mortar for the job and allow it to rest the amount of time specified by the manufacturer. Starting in a corner, apply the mortar to the backerboard with the appropriate-sized notched trowel, working in an area no larger than 2 to 3 square feet.
3. Begin laying tiles by working out from the corner. Press a tile firmly into the mortar and wiggle it slightly as you press down. Continue filling in tiles on either side of the first tile; use plastic spacers to create even gaps for the grout. Once all the full tiles are in place, cut tiles as necessary to fill in. Then remove the spacers.
4. Once all the tiles are set, remove the edging-strip guide and add the edging pieces. The simplest way to do this is by “back buttering”— smearing some thinset mortar on the back faces of the edging tile as if buttering a piece of bread. Then position the edging so it butts up against the full tiles, press down, and wiggle to set.
5. Allow the mortar to set up overnight. Then mix up sufficient grout to fill in the gaps between the tiles, following the manufacturer’s directions. Apply the grout with a grout float held diagonally to the surface, forcing the grout between the tiles. To remove the excess, hold the float at a 45-degree angle and scrape the surface.