Dry lay several of the tiles for your installation out on the floor of the bathroom with space to simulate the grout joints. Measure the distance between the tiles to get an idea of the layout, then transfer those measurements to the wall. While the niche is designed to be placed between two wall studs, you want the top and bottom sections to line up with full tile.
Mark your wall with pencil lines representing the grout lines, so you can see where the horizontal grout lines will be once the tile is installed. The most common method is to start with full tile on the bottom of the wall against the pan, then terminate wherever full tile falls above the shower head. This is usually around 7 feet in residential construction. Use the level, pencil and tape measure to help you lay out the shower, using your dry laid tiles as a guideline for where the horizontal grout lines should be.
Adjust the height of the box on the wall according to the layout of your tile dimensions. If your box or your tiles do not match up in terms of size, you want to center the box horizontally so that there are same-size cuts on either side. You are locked in vertically, because the niche goes between two wall studs.
Place the pre-fabricated niche box on the wall and draw its outline with a pencil after you determine its height. Cut out the backer board with a utility knife to create an opening for the niche. Mix some thinset mortar in a bucket with your drill and paddle attachment. Spread thinset onto the sides and back of the niche with the notched trowel. Place the niche into the opening and press it firmly in place. Install the tile for your shower.