Take your two boards with the layout marks on them and set them across from each other so that the marks line up.

Lay your two pieces with the layout marks on them facing each other and lay your six 6' - 9" pieces in between, roughly in the position of the marks.

Pre-drill two holes from the outside at the ends of the board to avoid splitting as shown on the second photo of this step. (The pre-drilling is only necessary at the ends of the board, not in the middle.) Screw (or nail, if you choose) the pieces into position.

ADD IMAGE SHOWING THE DRILLING AT THE ENDS. THE SECOND IMAGE SHOWS SCREWING IN THE MIDDLE, SO THE LANGUAGE AND THE PICTURE DON"T LINE UP RIGHT NOW.