I grew up making pop-ups, learning from my mother, Joan Irvine, aka. The Pop-Up Lady! She is the author of best-sellers such as How to Make Pop-Ups, Super Pop-Ups, Holiday Pop-Ups and has other How To titles such as Make it with Boxes. Her books are all over the world in several languages.
She has updated and moved her site. If you would like to learn how to make a pop-up, you can visit her site http://www.joanirvine.com . An excerpt from her book, shows you all the steps to make a pop-up mouth (illustrated by Barbara Ried).