Cutting the tile trim is like cutting a picture frame, all of the joins have to be at 45-degree angles. Double check your measurements by placing the trim across the length and height of the niche to make sure they’re correct. Place the trim in the mitre box and cut them with a hacksaw. After cutting the trim, use a file to create a nice smooth edge. Repeat this process to cut the three other pieces of trim, making sure that the 45-degree angles at each end of the trim are opposing.