Use a stud finder to locate the wall studs. Hold the stud finder in front of you and wave it back and forth. Wait for it to indicate the location of the studs and then mark the area with a pencil.
Measure where the top of the medicine cabinet will go. It is a common issue to hang a medicine cabinet too high for every day use. Ideally, the top of the medicine cabinet should be at eye level. Mark this spot on the wall with your pencil.
Have a friend lift the medicine cabinet into place. She can hold it for you while you attach the screws. It is very difficult to try to do this on your own.
Insert the mounting screws through the back of the cabinet, into the studs, with a cordless screwdriver. The screws should be at least 1 inch long to get a good bite on the wood of the stud. If the cabinet is heavier than 10 pounds, go up to 2-inch screws. Tighten the screws until the head is flush with the back of the cabinet.