Measure the width of your closet using a measuring tape. Measure from wall to wall in the interior of the closet where you want your shelving to go. Most shelves go from one side of your closet to another, though you can make it shorter if you wish. A shorter shelf requires L-bracket supports.
Mark the measurement you found in Step 1 on a length of 1 x 12 inch board using a pencil. A piece of 1 x 12 inch board makes a shelf that is 12 inches deep. If you want a narrower or wider shelf, choose another width of board.
Cut at the pencil mark that you made using a saw. At this point, you may mount the cut piece in the closet in the way that you choose.