Take a piece of A5 card. Make 3 small marks evenly spaced along the long (21cm) edge at the top and bottom of the card, 5.25cm, 10.5cm and 15.75cm. Mark halfway down the shorter (15cm)edge, 7.5cm on one side.
Score diagonally from the top first to the bottom third mark and from top third mark to first bottom mark. Score vertically between the top second and bottom second marks, do the same on the third marks, see attached photo.
Cut from the middle mark on the left edge to the first mark at the top and same again on the bottom. Make the folds as shown in the photo and decorate as required. I have described as best I can, it sooo much easier to demonstrate than describe the process!!
Look at http://www.art-of-craft.co.uk/acatalog/Spring_Card_Collection.html for the template available.
Good luck and have fun making your diamond spring cards.