Measure the width of the shelf or shelves you'll be applying the trim to.
Place a length of wooden trim into a miter box. Cut the trim to match the width of the shelf, using the guides in the miter box to make a 45-degree angle from the back to the front of each piece, sloping inwards towards the center of the piece of trim. Sand the cut edges of the trim until they are smooth.
Apply a bead of wood glue across the back side of the trim. Place the trim against the front of the shelf and nail it to the shelf front with 3d finishing nails.
Place a nail set on the head of each nail and tap on the nail set with a hammer until the head of the nail is below the surface of the trim. Apply a small dap of wood filler in each of the holes above the recessed nail heads.
Sand the wood filler smooth when it's dry. Paint or stain the trim to match the shelves.