Using your computer's word-processing program, design and print off the text for your card. I went the simple route and just chose the word "love" (colored in red), but you can be as elaborate as you want. If you have super-cute handwriting (which I don't), you can do this part by hand.
Put some glue on the back of your heart, then stick it to the front of your card; this will hold it in place while you sew and prevent you from tearing out your hair in frustration.
Using your needle and thread, do a simple stitch around the heart. Extra bonus: use a pushpin to poke holes around the perimeter of your fabric heart beforehand to create a template for sewing. I didn't do this, but it would have been easier to sew if I had.
Add a personal note inside your card, then find an envelope that fits and send your card to a special someone.