2. At this point, we'll assume you've tiled the rest of the wall leading up to the general proposed shelf area. If possible, you want your niche to fit seamlessly in your tile design. In other words, the bottom and top of the niche should start and end at the grout lines. Eventually you'll cut a rectangle that is about 3/4" larger (the orange lines above) than where your grout lines meet in order to give space for the backerboard and tile that will later be installed inside the niche.
When you go to place the kerdi into the niche you should fold in the sides as it will be placed into the wall. By folding the flaps over and in before placing the kerdi in it will help get it in the right place with a minimum of mess.