Measure the first horizontal edge of the backsplash, where it meets the wall. Mark out that measurement onto a piece of trim, with two pencil marks and the measured distance between them.
Set the trim on the miter saw, with one of the marks in front of the saw blade. Turn the blade so it’s pointing 45 degrees outward, away from the trim. Cut the trim.
Move the trim so the second mark is under the blade. Swivel the blade to 45 degrees outward in the other direction, so it’s again pointing outward. Make the cut.
Hold the trim along the top of the backsplash, against the unfinished edge of the tile. The mitered cuts should angle out over the ends at the two upper corners. Secure it there with your pin nailer, shooting a pin in every 12 inches.
Measure one vertical edge of the backsplash. Cut the upper end in the same manner as before, at 45 degrees pointing outward, but the downward-facing end at 90 degrees (straight across). Set it in place with the pin nailer, so the mitered top end of the trim butts to the mitered end of the horizontal top piece. The straight-cut bottom end should sit on the countertop.
Repeat the process to cut and hang the trim for the other vertical edge of the backsplash.