I recalled seeing these doily crafts in a Martha Stewart issue. You turn your heart upside down, fold the two sides in a little, fold up a little of the bottom and then bring down the point to create an envelope. Her site shows an example of how to do it, too.
I used gold wrapping paper with notes on it as the inside and glued it on. I printed out on the laser printer the "Valentine, You Make My Heart Sing" note for the insert and glued that on. I cut both with Fiskars special edged scissors.
Aidan wrote his name on Dresden trim bought from our local Art Store, Artist and Display (the gold paper came from there, too). We glued them on. After the glue dried, we folded the cards, added a heart sticker to keep them closed, and as only a 4-year-old boy could, put car, train, and airplane stickers on the back.