A process for placing elements of a self-stiffened panel of preimpregnated composite. The panel includes a base skin and a plurality of U-shaped stiffeners, with an interfacing structure including a nail and a nail-head. The process includes draping plies to form the base skin; placing and shaping at least one reinforcing ply on a set of punches; forming half-nails; placing a half-nail on the sides of each punch covered by the reinforcing ply; positioning the punches side by side in inverted U-positions; placing the nail-heads; compacting the set of punches; turning the punches 180 degrees; positioning the set of punches on the base skin; and withdrawing the set of U-shaped punches.