Take an A3 paper, fold an angle to the opposite side's middle so you draw kind of a diagonal from a corner to the middle of the further opposite side (the short one).
Do the same symetricaly, it will overlap on a few centimeters.
Fold the long strip in it's center perpendicular to the long side.
Make a preparation diagonal fold before you transform it into a square by folding in the extremities toward the center.
I recommend to use a paper to fold so your nails don't leave traces nor 'paint' the ink all over ... if you're using white paper mmmmmm so beautiful white paper ...