You can find directions for these all over the internet, and then just tweak the measurements to suit your needs. Here are the measurements I used to fit in a standard card envie:
- Cut base to 4" x 12"
- Score at 1-1/4", 2-1/2", 4-1/2", 6 1/2", and 9-1/4".
- Alternately fold mountains and valleys.
And because I couldn't stop myself from taking pics of this mermaid, here's a few more:
And the envelope seal: