I recently found this really fun and super easy tutorial for making these 3D flower pop-up cards.
All you need is one sheet of 8 1/2 x 11 inch paper, a pair of scissors and some double-sided sticky tape. Note: I used cardstock. It is harder to work with; but I thought the results were more professional.
If you'd like to make the cards following the same tutorial I used, then go here. c.