Cut the sides of the rectangle, which should be no less than 16 inches wide. If your t-shirt is less than 16 inches wide, do not cut the sides of the rectangle. Instead, make one cut in the circular piece of fabric you have. This will allow it to open up to twice the width of the t-shirt. Measure 16 inches in width and cut the fabric. When finished, you should have a rectangle of fabric 16 inches by 8 inches long (for an adult's mask).