USE A MAGNET ON A STRING
Finding studs with those expensive store-bought gadgets is a bit like using a divining rod to sniff out groundwater. It’s hit or miss at best. For a better way that’s also much cheaper, start with a small, strong magnet. (Suppository-shaped cow magnets, sold at farm supply stores and by large-animal vets, work exceptionally well.)
Attach a couple of feet of string to one end using tape or glue. Dangle the magnet alongside the wall at about the height you want to drive your nail. Slowly move your hand along the wall, raising it up and down if you get no response. The magnet should be drawn to either a framing nail or a sheetrock screw in a stud. Mark the spot and use a level to project a line straight up or down to the height you want. Then drive the nail home. This works well on walls covered with drywall but may be less successful on walls covered with plaster. It won’t work at all if the plaster is on metal lath.