The other day I needed to make some circles for a rocket project so I built a router table out of a piece of plywood and a few 2/4’s for legs, then measured out from the router blade for the radius I needed and put a screw down through the table. I removed the screw and put it back into the same hole from the bottom side so the point would stick up just a bit. Then the trick is to turn on the router and slide a slightly oversized square to where it just barely touches the blade and press the square down on to the screw point. Needless to say this is the tricky part and must be done carefully to prevent the piece from launching across the room. Also the screw point needs to hit about the center of the square. Then the square gets rotated counter clockwise slowly to cut out the circle. Later the dent made by the screw will make a convenient center mark for resizing the circle smaller and/or cutting out a concentric hole .