Before building the frame, sand the plywood piece for the top. Start with 80 grit sandpaper, wrapped around the sanding block or scrap piece of wood, and sand both faces of the plywood. Then, sand again with the 120 grit sandpaper and finish with the 220 grit sandpaper.
Brush away the sanding dust with a tack cloth or old paint brush.
Now, it's time to apply the edge banding to cover the exposed edges of the plywood. Starting at one raw edge, cut a piece of banding to fit. Apply the banding with the adhesive side against the raw edge of the plywood and use an iron on high heat to activate the adhesive.
Once the banding is adhered, use an edge banding trimmer (or craft knife) to cut the edge banding flush with the faces of the plywood, if needed. Cut away any of the banding that overhangs the ends with sharp scissors.
See this blog post
for an in-depth tutorial on applying edge banding!