Brush a stud finder over the wall where you plan to hang the medicine cabinet. Mark the location of the studs.
Draw a line directly up from the sink faucet if you plan to center the medicine cabinet over the sink. Set a level against the wall through the middle of the line to account for any variances in the wall. Adjust the line if necessary so it is level. This is the plumb line.
Raise the medicine cabinet into position with a partner, centering it on the plumb line. Make two marks on the wall to indicate where the bottom of the cabinet should go. Make another mark on the wall to indicate the top of the cabinet. Intersect this line with the plumb line.
Hold the cabinet in place and mark the position on the wall for the screws. The medicine cabinet should have two or three holes already placed inside for the screws. If possible, position the holes so they lie over studs. Drill wood screws into the studs, but don't tighten them completely. Once all the screws are in place and the cabinet is centered, tighten them.