A template isn't really needed unless you are making the same cut several times. Just use a straight edge as a guide and clamp it in place on all four sides of the cut. The guide should be clamped at a distance equal to the distance from the base of the router to the edge of the router bit. Then just plunge the bit through the panel and follow your guides. Take it slow and let the bit do the work. You'll be surprised at how easily the router bit cuts through the plywood. IMO this is far more efficient that rough cutting the hole then finishing the edges in a separate step but that will certainly get the job done as well.
Make sure to shoot us some pictures when you're done.
