I am going to take a guess with this ruler. You lay your fabric out the same as when you are doing straight cuts. Lay this ruler with the line closest to the number on the fold or edge of your fabric closest to you, but make sure the left edge (or top of the ruler point) doesn't go off your fabric on the left side. Cut the width of what you want your diamonds to be UNFINISHED. Start at the number 1 and if you want 2 1/2 UNFINISHED diamonds next cut is 2 1/2, next cut at 5. Then you have to FLIP the ruler and line up the line closest to the numbers on one of your cut edges and cut the same way. All of these cuts this way will be on the bias so you will need to starch to help with stretching before cutting.
Good luck and let us know how you make out....