Determine the appropriate location for the smoke dectectors in your home and cut in boxes in the upper walls or ceilings. When installing in the walls, usually the detectors are mounted around 12" below the ceiling. Use a stud finder to locate the wall studs so the box doesn't end up on a stud.
Hold the cut-in box up on the wall and use a level to mark the outside of the box. Mark this in pencil, not marker. If you are installing the boxes in the ceiling, locate the ceiling joists and cut next to them. This will give you a nice box support. After all are marked, use a drywall saw to cut the holes in the drywall.
