Most of my tools are older machines that I have rebuilt and repaired. As such I can't buy off the shelf parts for them. I needed to make very accurate circles and was tired of going through the process of setting up the table saw each time to make one-off circles in thin sheet goods. This is how I quickly mocked up a custom circle jig using a method that would work with any router.
1. Start with a thin piece of plywood the same width as the router base.
2. Mark the exact centerline of your wood.
3. Take off the router base and use it as a template to mark out mounting holes. Remember to countersink the holes so the router mounting screws sit flush or below the surface of the wood. Do this on the side OPPOSITE the centerline you marked in step 2.
4. Screw the template to the router UPSIDE DOWN.
5. Install the bit you are going to use to cut and measure accurately from it down the centerline you marked in step 2. Mark out several inches (I did 4,5,6,7,8,9).
6. Remove template from router and use a straight edge to carry your lines out to the end of the board marking them as you go.
7. Reattach template to router base in place of the factory base with the centerline up.
8. Drill a small accurate hole 1/2 the distance (radius) of the circle diameter you want to cut
9. Best to start with a circle close to the finished diamiter so your router isn't making a full deep cut.
10. CIRCLE ALL THE THINGS!
Other instructables of interest:
Cutting circles on a table saw: https://www.instructables.com/id/Large-Slab-Table-S...
Rebuilding the router you see in this video: https://www.instructables.com/id/Router-Circle-Jig-...