To make rounded trim for the undermantel of your fireplace, clamp your ruler 1/8 inch in from the rounded edge of a piece of stripwood, at least as long as the width of your fireplace. The piece used for the bashed hutch fireplace is cut from the piece of wood with the rounded edge which was once the top of the hutch. Run your craft knife along the ruler carefully to cut free your half round trim.
If you are scratch building a fireplace, you create rounded trim by sanding a length of stripwood to a half round profile, clamping it with a ruler, and using the ruler as a guide to cut the rounded trim free.
If you are building your fireplace from bookboard, Cut a narrow 1/8 inch wide strip to use as trim.