Measure the area between the counters or between the counter and the wall with a measuring tape. Write down the width and length measurements.
Trace these measurements onto the plywood with a pencil. These are your cut lines. Work slowly to keep your lines even.
Cut the plywood with a circular saw. Trim the plywood along the cut lines. Do not force the plywood through the saw to avoid creating jagged edges.
Attach one end of the hinges to the counter with a cordless screwdriver and screws. Screw the hinges into the underside lip of the counter.
Install a collapsible support bar with a cordless screwdriver. Place the support in the middle of the hinges. Screw it in to the underside of the counter edge.
Attach the hinges and support bar to the plywood with a cordless screwdriver. Hold the plywood in place and attach both hinges and the support to the bottom of the wood. Test it to make sure it folds properly.
Finish the flip-up counter to your preference. If staining the wood, sand it down first, and apply a coat of wood stain and then a coat of polyurethane. If painting, prime the plywood first and then paint it with an interior latex paint.