Step 1
Remove all traces of the old shelf support which could be in the way of a new support. Measure the interior distance of the shelf from the inside of one shelf wall to the other. Transfer this measurement to the dowel and mark the distance with a pencil. Cut the dowel to the appropriate length using the saw.
Step 2
Place the dowel vertically in the vice and tighten it so that it is fixed in place. Using a drill bit with a diameter slightly smaller than the wood screws, drill a hole in the center of the end of the dowel. Flip the dowel over and drill a hole in the opposite side as well.
Step 3
Position the dowel within the shelf until you have established the correct position for ideal storage space and support. Mark the location of the dowel on the outside of the shelf sides with the pencil. Remove the dowel and predrill a hole in each shelf wall. Attach the dowel in place by screwing a wood screw through the outside of each shelf support and into the dowel.
Step 4
Test the position and strength of the new shelf support bar and if necessary repeat the process to add one or two more dowel supports to the shelf system.