Drag a stud finder across the wall to locate the studs within the closet. Follow the directions for the stud finder for proper use. Mark each stud location with an "X" on the wall using a pencil.
Hold the level to the wall so that the tool is leveled horizontally at the height you want the shelf. Draw a straight line crossing the studs for the length of the shelving using the level as a straight edge.
Mount the first support bracket, included with the hardware from the shelving unit, to the stud closest to the end of the location for the shelf. Drive one of the included screws from the hardware into the wall with the drill and screwdriver attachment so that the top of the support bracket is flush with the horizontal line on the wall.
Hold the level against the side of the bracket to verify that it is plumb. Drive the remaining screws through the support bracket into the wall stud.
Mount the remaining brackets in the same manner, spaced no more than 48 inches apart. Each bracket needs to attach to a wall stud.
Place the shelf on top of the brackets and secure the unit to the brackets with the included hardware. Metal shelving will clip in place with plastic clips. Wood shelving will require that you drive screws through the support bracket into the shelf.