Empty the mounted shelf to which you wish to add shelf dividers. Measure the width and height of the shelf with the measuring tape. Gather both the height and width of the shelf dividers and write them down.
Draw the shelf dividers on the plywood sheet with a pencil, using the height and width measurements. Each shelf divider must measure the width by the height. Draw as many shelf dividers as you please for the given shelf. Use a ruler to make the lines straight. Cut out the dividers and sand the freshly cut sides.
Locate the half-inch ends along the width-length that will line up directly with the top surface of the shelf. Space evenly three to four pencil marks along the end of the dividers. Drill half-inch deep holes using the marks for guidance. Insert one wooden dowel into each drilled hole. Repeat this step for each divider.
Place one shelf divider on the shelf, so the wooden dowels sticking out of a divider touch the surface of the shelf. Use a pencil to mark where the dowels touch the shelf, so you know where to drill holes to attach the divider. Drill half-inch holes on the shelf using the marks as guides, but avoid drilling through the shelf. Push the divider in place so the dowels attach the shelf with the divider. Repeat this step for each divider you wish to install.