Use the stud finder to locate the studs in the wall. Plan on placing your recessed medicine cabinet between the studs, unless you plan to do a lot of extra work such as cutting out the studs and bracing the wall opening. If the recessed cabinet doesn't fit between the studs and you don't have the skills to brace the opening, install a wall-mounted cabinet instead.
Mark the spot on the wall where you plan to install the recessed medicine cabinet between the studs. Use the drywall saw to cut a small hole inside the marked area.
Look inside the hole to make sure there are no plumbing vents, pipes or electrical lines to interfere with the installation of the cabinet. If there are, reconsider the wall-mounted cabinet.
Cut a hole in the wall between the studs and within your marked lines if there are no obstructions. Do not poke a hole in the opposite wall as you cut.
Insert 2x4 lumber the length of the medicine cabinet inside the hole. Nail it to the studs on both sides of the hole.
Slip the medicine cabinet into the hole. Level it. Make sure the door has room to operate properly. Using the pre-drilled holes that come on the medicine cabinet, screw the screws into the 2x4 pieces.