Tina,
Jezabel's instructions are correct. I have one additional suggestion. Once you have "squared" the rough or bruised edge of your fabric, use two rulers. Use a larger one to the left and then butt a smaller ruler against it to the right, making sure it is 4" toward the top of your cut as well as at the bottom next to your belly. This way you won't disturb the squared edge.
Also, the lines (next to your belly) on the larger ruler should line up with the folded edge of your fabric.
Hope this helps. Suz