Download the printable here. There are four different designs on one sheet. Best printed on 8.5 x 11 matte photo or heavy cardstock paper. Below's a little tut just in case.
Cut the valentines out. If it's printed on matte photo or heavy cardstock paper, lightly score the lines with an exacto knife. This makes it easier to fold.
Squeeze the valentine open. Fold down the flaps on one end. It should fold down easily, especially if you scored it. If you need to you can glue or tape this end so whatever you put in it doesn't fall out.