Drill holes into the circular end of the jig to correspond with the holes in the sub-base. Use the sub-base to guide the drilling.
To use the jig to cut a circle, draw a line on the work piece from its center to the end of the desired radius of the circle. Center the router atop the end point of the radius. Drive a nail through the appropriate hole in the jig that corresponds with the center of the work piece (image 1). Set the router to the proper depth, and make a slow pass through the work piece, using the nail as a pivot point (image 2). Make deep cuts in several passes, lowering the router bit slightly after each pass.
The second jig for cutting curves and circles with a router can be made from scraps that are around the workshop. Like the jig described above, it's attached to the router's sub-base.
Drill holes the same size as the dowels into one long edge of the 12" x 1x4 wood block. The holes should be as far apart as the diameter of the router's base.
To use the jig to cut an arch or a circle, nail the center of the block to the center of the desired circle (image 1). Slide the router to the appropriate spot on the dowels to correspond with the end point of the desired radius. Set the router bit to the appropriate depth. Slowly swing the router through the work piece, using the wood block as a pivot point (image 2). Repeat the process as necessary, slowly lowering the bit with each pass.