You could also make one for your anniversary, or use it as a more personal way to present a gift card you've bought a couple as a wedding gift.
1. Cut out a red heart from felt, paper or card using the heart template from this tutorial.
2. Stick the heart onto the front of a mini glassine envelope using double-sided tape. Make sure you leave enough room at the top of the envelope to fold it over.
If you can't get hold of any glassine envelopes, you could make your own see-through envelope using tracing paper. Search online for an envelope template you like, cut and fold the tracing paper to form the envelope and stick it together with double-sided tape.
3. Insert the love letter / tickets / gift card / etc into the envelope.
If you don't want to fold important tickets to fit the envelope (or, like me, you're a bit fussy and don't like how the colour of them clashes with the rest of the card) use a photocopier or a scanner to make a black and white copy of the ticket and add that to the envelope instead.
4. Fold over the top of the envelope, and position it roughly in the centre of your blank card - I used bright red card, but white would also look great and make the red heart stand out even more. Then stick the envelope in position with a piece of patterned washi tape (Japanese masking tape), torn at each end.