Position the valances around the edge of the bottom side of the cabinet, so that they are about 1/2 inch in from the outer edge. Cut the valances to fit using a miter saw so that they form a perfect U around the sides and front edges, but not the rear edge.
Sand all of the surfaces of the light valance lightly with fine grit sandpaper. Wipe it smooth to remove all of the sawdust.
Determine the exact paint or stain color of your cabinetry using color matching or other techniques, or contact the original owner or builder for that information. Coat the sanded valances with smooth, even brush strokes of paint or stain, applying in the same direction as the wood grain.
Allow the first coat 2 hours to dry, then apply a second coat in the same direction and manner. Repeat until the dried surface of the valances has the same shade of coloring as the original cabinetry.
Apply a thin coat of sealant, following the same procedure as the stain or paint application. Repeat to add three or more coats of sealant, stopping when the dried valance seems to have the same gloss as the cabinetry. Allow the valances to dry overnight before installing.
Reposition the valances about a 1/2 inch from the outside edge on the bottom surface of the cabinets. Have an assistant hold them in place and drill down through the base of the cabinet into the valance boards every six inches. Be careful not to drill through the valances or out the sides of the valances.
Set 2-inch wood screws in each hole and tighten them securely to fasten the valance boards to the cabinet bottom. Sink the screw heads slightly deeper than flush with the cabinet surface, so that they do not hit against dishes stored in the cabinet.