I use it in a vice, but you could also screw it to a piece of wood and clamp it to your worktable. The workstops (bottom center) set the corner to flush, and the notches in the front allow the router to run over the workpiece seamlessly. The little roundovers on the guide pieces form a nice finished radius on the outside corners with a bearing guided straight bit.
Here are the two pieces. 3/16" plywood was doubled to get it to the 3/8" required, and still let me cut the roundover on the edges.
Together, you can see the gap for the corner.
A corner ready to be trimmed. The jig is laying on it's side for clarity.
And a trimmed corner next to a stock one. This trims to exactly 3/8", so it should be perfect by the time you add your covering. I haven't decided yet whether to square off the inside radius or not, I'll wait and see how they fit over carpet.
