This picture doesn't quite look right. It is actually flat. This is how it looks when you take it from the envelope.
Then you gently pull up the row of hearts.
These are made using this very clever tutorial from Paper Cookies. Go and have a look at her design - it would make a great valentine. It is very simple and easy to make. If you like I can even e-mail you the word document that I used for the box part which has all the fold lines marked on it. The slide up picture I made using the hearts and circle design set on Publisher.