Robert Sabuda, the pop-up book master, has some great tutorials on his website to make pop-up greeting cards for your Valentine. They even include printouts for mistake free assembly! I would encourage you check out Robert’s entire pop-up tutorial library. It’s pretty fabulous.
The Pop-up Card Designer is a Windows application, which generates unfolded patterns of pop up cards. Using this software, you can print out a pattern and use it to create your own pop-up card.