The card should print out the be 9.5 inches wide. We're going to make a score along the middle of the card so it will fold more easily and cleanly. If we didn't do this, and tried to fold it, the ink would most likely crack and different stress marks would appear on the paper.
Measure out to the center of the card on the top and bottom, and make a few marks to line the ruler up with. Go ahead and make the score with your X-Acto knife.
If you're not comfortable with how much pressure to exert (as to not cut the paper), you can use the backside of the knife, rather than the blade.