Measure the top of your air conditioner. Pencil the outline of the unit onto a piece of plywood, using a ruler to make straight edges. Add an extra margin of one to two inches around the border and make a second, larger outline.
Use a circular saw to cut the plywood to size, following the larger outline you have drawn. Sand the sides to remove any splintered or rough edges.
Paint or varnish the plywood on all sides. Let the paint or varnish dry completely, according to the manufacturer's instructions. Sand the entire surface with fine grit sandpaper if you prefer a smoother finish. For the final round of sanding, moisten the sandpaper with a bit of water.
Set the plywood over the top of the air conditioner condenser, lining it up so fully cover the unit. Set a brick at each corner to weigh the plywood down. Paint the bricks the same color as the plywood, if you so wish, to create a more uniform look.