Install a wooden strip across the bottom area of the installation to continue the line for the backsplash tile across the height of the countertop. For the area between two base cabinet sections, cut the wood to fit with the hand saw. For an area extending beyond the side, such as behind a fridge, measure the piece of wood the length of your installation, cut it, mark a level line and install the board on the line. Screw the boards in place with a drill and screws.
Install the tile or natural stone across the rest of your countertop and carry the installation across the filler boards according to whatever pattern you are installing. Spread mastic onto the wall with a notched trowel above the filler strip of wood and press the tiles into the mastic firmly so they bond with the adhesive. Use tile spacers and wedges to keep the joints uniform.
Leave the board in place for a minimum of 24 hours to allow the mastic time to set up and hold the tiles in place. For best results, do not remove the board until after you have grouted the installation and given everything 72 hours or more to cure so that you eliminate all risks of anything moving if the mastic hasn’t set, such as in cases of humid weather where the mastic takes longer to cure.