Begin with an 8 1/2" x 11" pieces of construction paper or card stock.
Card stock works best and easily obtained from any office supply store. You may also begin by using a manila filing folder. This maybe easier for very young pop-up makes (if using a manila filing folder skip to step 3).
Fold the construction paper in half to form a card.
Draw two parallel lines of equal height towards the fold of the card. The lines do not have to be perfect as long as they are the same height.