After fining the studs, mark the area where the tiles need to be cut with the china marker. The marker will not stain or discolour the tile, so you can make as many marks as you wish.
Studs usually don't meet at the grout lines, so you'll be mostly cutting directly into the tile. Carefully lay out an area between the studs. Ours will also have a stud in the centre. We worked with a space that was 32 ½ inches wide and 42 inches tall, approximately 3 ½ to 4 feet from the floor of the shower.
After marking the area, recheck your measurements using the level.
Many homes are not exactly square and level, so getting the square as close as possible is fine in many cases.
That 32 ½ x 42-inch rectangle will be the size of your shower nook. Since most homes are built with studs with 16 inches on centre between them, this should give you a secure installation.
Also, the specific location you want your new shower nook may require studs to be removed. This can be done, and the studs cut out can be used to frame the top and bottom of the nook. These supports are also called cross-members.