Here we have applied contact cement to the front inch of the header and about three inches of the top. We have pulled a uniform amount of tension in the top, so that it is even across the whole thing. You will start out pulling more on the outer edge and gradually lessen the tension as you work your way to the center. Pulling the same amount all the way across will get you a super-tight center, and loose wrinkles at the outside. You should end up with about the same amount of excess top material across the entire length of the header.
Once you are happy with the pull, press it down firmly onto the header.