This is my first time to make a "Diamond Fold Card". I wrote down the steps.
30cm X 10cm paper (or 12inch X 4inch)
Bend the paper from the middle to make it looks like triangle.
Bend it to other side.
Bend it to four part as the picture.
Follow the line you bend, then the Diamond card finished.
P.s. My English isn't good enough to describe the steps. You can see the pictures to make it.