a few sheets of paper
sheets of cardstock, 1 per Valentine
& whatever else you may want to add: stickers, stamps, confetti, glitter, etc.
Start by cutting out a large heart from a normal sheet of paper. I used the elementary school trick of folding my paper in half (hamburger style) to make the sides even. Then, fold your envelope to test the size of your heart. With the point of the heart towards you, fold the sides toward the middle (1). Then, fold the top down (2). Rotate the envelope 180 (3) and then fold the point down to cover the flap (4). There's your envelope (5)! My first heart had too deep of a center V, and a too shallow bottom point, so the envelope didn't quite work when I folded it. Keep trying different heart shapes until you find one you like.
Using your internal paper (I used red construction paper, but feel free to go crazy with colors or patterns), cut out rectangles that will fit into the center of your heart envelopes. I traced around a folded envelope as a guide, then just trimmed down the sides to make it fit. Then, write out your messages. Glue your message into the center of the heart; I used a spray adhesive so that there wouldn't be any wrinkles, but any glue or tape will do the job.
Address your Valentine (do this before sealing your envelope). I put my return address on the back because the envelopes are kind of small & I wanted them to look clean. I recommend running the back of a knife over the creases in the paper to help them stay folded. I used a little black heart stamp in front of the address and a heart sticker to seal the envelope to give them simple details. The details are what personalizes it, so add plenty of your own inside and out.