Set each vertical piece in place along the sides of your locker, one per corner. The long (1 5/8") side of each vertical should be against the longer (~17") dimension of your locker.
Place one shelf on the bottom of your locker to lock in the bottoms of the vertical pieces.
Add shelf clips at various heights then install your shelves. Don't get too close to the top or you'll find it's too easy to dislodge that top shelf. Staying one or two notches below the top seems to work fine (or, secure the top shelf to the clips).