I am making a different quilt with diamonds and what I do is sew ny strips together then place my ruler across the 3 strips, cut the 45degree angle. In your case, since you want 1.5 inch pieces, you then place your ruler on the 45 degree angle you just cut at the 1.5 inch line on your ruler and then cut your next piece. Continue to do this across your entire strip. You will then have all your diamond shapes. Try it with scrap fabric so you can get how the process works. Hope my explanation makes sense.