Once you've installed the tile niche, the next step is to install the tile trim around the edge. It's important to select the tile trim according to the thickness of your tiles. When you go to buy your trim, it's a good idea to take a piece of tile with you to get the right size.
Use your tape measure to measure the length and height of the tile niche. Take into account that you will be cutting the tile trim at 45-degree angles. Use the marker or pencil to mark these measurements on the tile trim.
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.
Put the bottom piece of the tile trim into place. Use the painters tape to secure it and make sure it's level. Repeat this process for the trim on the sides and top. Don't nail in the trim, it could compromise the waterproof membrane. Now you've installed the trim, you're ready to start tiling the niche.