Cut the wooden molding into strips to fit the length of your sign (70 inches).
Lay the aluminum or plastic sheet flat on ground. Glue the first strip of molding lengthwise across the top edge of the sign with the "lip" facing downward. Use the plywood to help you keep the line straight if necessary.
Measure 8 inches down the sign, and glue another strip across the board with the "lip" facing the other direction. This is your first row of letters. Measure another inch down, and start a second row.
Repeat the process until there are ten 8-inch rows, spaced 1 inch apart. Make sure each row has the "lips" of the molding facing toward each other.
Allow the glue to dry. If you wish, you may repeat this process on a second sheet to make a double-sided sign.
Glue the aluminum/plastic sheet to the wooden frame. You can order 8-inch letters to fit your board at http://www.letterperfectinc.net.