Measure 24 inches on the 1-by-6-inch board. Mark it. Use your saw to cut it straight across at your mark. This will be the back of your letter holder.
Cut four pieces of wood on the remaining 1-by-6. Each piece should be 6 inches long. These will be the holders for letters.
Cut two piece of wood for the sides from the 1-by-4-inch board. They should each be 24 inches long. Attach the sides of the letter to the back with wood screws.
Attach the dividers to the back of the letter holder between the sides. Tilt the dividers forward so a letter will stay when it is placed in the slot. Use wood screws screwed through the side of the letter holder into each divider.
Sand all the edges. Leave the wood unfinished if you prefer the natural look. You can also apply stain and varnish for a more finished look, or even apply paint.