Locate the wall studs with a stud finder or a hammer and nail. Then, hold the template that came with the safe between the studs. Using a level, line it up so that it is straight horizontally. Tape it in place.
Draw the square onto the wall, and remove the template. Cut out the square using a drywall saw.
Fill the space between the body of the safe and the studs. If your safe does not fit snugly between the studs, nail wood shims to the studs to fill the gap.
Insert the safe into the opening, and push it all the way in so that its front is flat against the wall. Using a drill or screwdriver, secure the safe in place with the screws provided.