Try these measurements out first on a piece of printer paper. I think they will work.
Step 1 -- Score 3.5" x 10.5" piece of patterned paper at 1.75" and at 8.75".
Step 2 -- (no change)
Step 3 -- On the back, mark the 5.25" mark on both sides.
Step 4 -- Take one end of the paper and fold it down diagonally at the 5.25" mark. Fold the opposite side down the same way.
Continue using the pictures in this tutorial.
Diamond Fold Card