This is a fun card fold and I particularly like it because people who receive it through the mail will easily know how it works as soon as they try to open it!
Decorating the card can take some imagination. But a simple way to make the triangle at the right stand out is to decorate the front of the card with a background stamp or overall texturizing, which makes the inside triangle stand out. For this card, I stamped both the outside and the inside, but used different embossing colors. Also, I did not stamp the whole of the inside of the card – just the diagonal made by the inside slit. So there is plenty of room to write a note!
Here are the instructions, with a larger, full-page set of instructions here.