Measure in 5 inches from the edge of one of the 20" boards. Fit the board with the 45 degree angle to the base board along the line you just drew. Using a drill bit, predrill and counter sink from the bottom of the base board into the angled board. You should now have a "y" shaped construction.

Now measure 8 inches in from the free end of the angled board and draw a line. Apply glue along this line then butt the next 20" board against it and clamp. You can shoot a few nails into it if you don't want to wait around for the glue to dry. Continue the process of measuring in 8 inches and attaching a new 20" board until there are 7 angled pieces attached together.