Since I had to figure out dimensions for an ATC, I kind of made up my own layout which is similar to snietje's but not what she suggested. The open photo was the easiest way to photo the card. It will pull out square with the base.
To resize the diamond: Cut a long strip the width you want the finished diamond. Start by marking the center of your strip and fold the diagonal diamond folds. Work backward from there to add the perpendicular folds. Then finally trim the paper to the desired width.