Make a sign on another piece of paper. Using large writing, make it a message to your valentine. Something as simple as "I love You" will work. Have someone take a picture of you holding the sign. Either make this soon enough before Valentine's Day that you have time to develop the film, or use a polaroid camera.

Cut out a large heart for the front of your card. You will want it big enough to have room to cut out the center of the heart and frame your picture with it. Glue your picture on the front of the card, and then glue the heart frame over that.