Step 4: Attach boards to backer, add finish
I carefully lined up the base piece with the backer board and used wood screws and glue to attach the two together. I did the same for the vertical pieces. I think if I had a biscuit joiner I would have used it to be sure all the boards were glued tight, but the screws worked. It is just kind of messy looking.
Attach the hanger while you are thinking about it and clean any glue that might have squeezed through off the front of the boards.
When the glue is dry, spray or brush on a finish. I used interior polyurethane. Wait, repeat, wait, repeat.
As always, read and follow the directions for your products.