With the first piece of trim attached to one side, continue the process with the front piece of 1x2 trim. Use another short piece of 45-degree mitered 1x2 on the opposite side to help you determine the exact length to cut the front trim piece. Then, mark locations for biscuit slots every 9-12 inches and cut the slots in the front piece of trim and front edge of the plywood. Insert glue and biscuits into the slots on the plywood and attach the front trim piece (with glue in the trim piece biscuit slots as well). Align the miters in the corner properly and hold the front trim piece in place with clamps or finish nails.
Continue the process by attaching the final piece of trim on the opposite side of the shelf from where you started, using the same steps as above.
TIP: You may wish to help strengthen (or maintain the integrity of) the mitered corner with a couple of finish nails, nailed from one trim piece (through the miter) and into the other trim piece. You can countersink the nails and fill the holes using wood filler.
ANOTHER TIP: Avoid using too much glue on the biscuit joints, particularly if you intend to use stain as the finish on the shelf (as stain does not cover glue very well). If you do experience glue run-out, try to clean up the excess glue with a slightly damp, clean cloth as quickly as possible.