Layout your pieces on a flat surface like a concrete slab or wooden floor deck. It helps a lot if this surface is completely level.

Lay your two longer pieces with the layout marks on them facing each other and lay your 6 shorter 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.