To fold the bowl, lay it face down on an edge, like a step, with the line of holes on the edge. Lay down a towel to protect the surface of the sign. Stand on the sign. Hit the metal with a mallet until it bends nearly 90 degrees. Repeat for the other long side.
Put the corner tabs on something like a fencepost and beat them to about 45 degrees. Do the same for the short sides. Everything should come together in a general bowl-like shape. The corner bolt holes should more or less overlap.