Pop Up Box Card Tutorial - by Wendy Cranford

https://www.youtube.com/watch?v=GiiGXr0UyEQ

http://www.luvinstampin.com/2014/02/pop-up-card-tutorial-fun-cards.html  Wendy's Blog with measurements

8.5 x 12
Score at
2 3/4  5 12  8 3/4  11   Cut off 2" from 8 1/2 side, making it 6 1/2 x 12
Score at 2 3/4 on short side
Cut the back flap at 2 3/4" x 6" and the other squares are 2 3/4" x 2 3/4"