Start with a piece of 1 x 8 pine. Cut it to 3.5' and sand it smooth. Paint it gray.
I had some scraps of masonite that measured 11" x 3.5" so I sanded them and cut them with a band saw to create an arrow shaped sign. If you don't have a band saw a sturdy piece of cardboard cut to the correct shape would work too.
I painted these black then white. This is a system I use when I want to distress the final piece and I want the black to show through instead of the color of the wood.