Before you prepare the mortar, clean the substrate of all dirt and oils. Place the tiles where they will go, with plastic spacers between them at the grout lines, and make adjustments as needed. Aim for a symmetrical look, with no narrow slivers of cut tiles.
If possible, plan to face tile cuts toward the countertop’s back edge, where they cuts will be covered by backsplash 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.
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.