I cut 4.5-inch squares for this tutorial but the same instructions can be used to cut any size square or rectangle.
Carefully move your strip so that it is horizontal, with selvages on the left. Keep the strip folded together. Line up the square ruler as you did in the "squaring up" section along the bottom edge of the strip, about 2-inches to the left of the selvage.
Now, again as you did in the "squaring up" section, place the long ruler against the left edge of the square ruler.
Carefully remove the square ruler.
Cut along the right side of the ruler and discard the selvages. The left and bottom edges are now at a perfect 90 degree angle.
Lay your long ruler on top of your strip lengthwise. Line up the left edge of the fabric with the 4.5-inch mark on the ruler. Cut along the right side of the ruler.
Separate the two fabric layers and you have two perfect 4.5-inch squares. Repeat these steps to make as many squares or rectangles as you need.