If you're looking to create a router template for a perfect circle, there are a few things you'll need to do. First, you'll need to find a piece of plywood or MDF that's at least 24 wide and 48 long. Next, use a jigsaw to cut out a circle that's 24 in diameter from the center of the board. Once you have your circle cut out, use a router with a 1/2 spiral up-cut bit to rout around the edge of the circle. Be sure to keep the router moving in a clockwise direction as you work your way around the perimeter of the circle.
Now that you have your routed edge, it's time to create your template. To do this, simply trace the outline of your routed edge onto some thin cardboard or Bristol board. Once you have your template traced out, use it to trace around the edge of your project piece (whatever it is that you're wanting to cut a perfect circle into). Now all you need to do is use your jigsaw (or other cutting tool) to follow along the line that you just traced - and voila! You should now have yourself a perfect circles every time!