Cut one of the poster boards in half width-wise.
Choose one of the shorter edges of one of the halves of the cut poster board. Measure a 1-inch margin on that edge using the ruler. Draw a line at that margin with the pencil. Cut 1 inch from a corner along that penciled margin.
Measure a 1-inch margin from the long edge that intersects with the cut you just made. Mark that margin using the pencil, just as you did for the shorter edge.
Fold along both the marked margins. Tape the corner where the folded “tabs” meet into a right angle, so the angle is secured in place. Repeat the process of marking and folding 1-inch margins, and taping the corner, on the other half of the cut poster board.
Measure a 2-inch margin along all four edges of one of the halves of the cut poster board. Trace along that margin with the pencil. Cut along the penciled outline – it should look like you’ve cut a window into the poster board. Repeat the cutting process on the second half of the cut poster board.
Stand the poster board halves on their long, folded edges, facing each other, with their other folded edges across from you.
Slide the uncut poster board between them, along the folded tabs – the uncut poster board should look like a ski slope, running toward you.
Glue each corner of the uncut poster board to the tab nearest it.