You need 3 matching pieces of card stock.
Cut 4 pieces at 12x6 inches
and 2 pieces at 5 1/2 x 5 1/2 inches
Of the 4 long pieces score one of them at 2 3/4 inch in and 3 inches in from the end (do this to both sides)
score the other 3 long pieces 3 inches in from each end
glue the 3 pieces that are scored at 3 inches on the ends together