Mark the borders of the curved garden bed by laying a garden hose in the configuration you desire, or by spray painting the borderline on the ground.
Dig out and remove all vegetation from the interior of the bed, using a garden shovel or rototiller.
Place rocks for the border along the marked line, at the edge of the now bare interior. Use rocks the size of a softball or larger, lining them up along the borderline.
Install plant materials of your choice in the interior of the bed. Place taller growing specimens toward the back of the bed if it is up against a fence or building, or in the center of the bed if it is accessible from all sides.
Mulch the soil with a 4-inch layer of smooth river gravel, or another small- to medium-sized gravel of your choice.