I have been making pop-up greeting cards for many years now. Once you understand the mechanics of making them they are very easy to do. Perhaps though, I should say, depending on the difficulty card you make, they can be easy to do. The critical thing one must do in making them is to cut them precisely, and fold them precisely.
A paper scorer, a metal ruler, paper glue [I use Elmer's], very sharp pointed scissors, and Exacto knife [or the equivalent], a self healing cutting mat [board], and card stock are necessary. I bought my paper scorers at Micheal's, but found my knife along with the cutting mat at and office supply store [either Staples or Office Depot].
The best book on making pop-ups is "The Pop-up Book" by Paul Jackson, for it shows how to make them, plus at the back of the book, shows a beautiful gallery of cards made by his students using the basic instructions he gave.
Another good series of books are the "How To Make Pop-ups" series by Joan Irvine. She has a book on general pop-ups as well as holiday pop-ups, and all have clear instructions.
I don't have any pictures of my pop-ups yet, but I have done pop-up bread machine [ a solid box] thank you note, valentines [stand-up hearts], butterflies, moths, birthday cakes [wedge and six-sided], presents [a box], reverse fold flowers, stars, bells, just to name a few.
I hope these books will inspire you to try making pop-up cards. People of all ages just love getting them, and it is fun to make and give them to others too.