I've done this using a dull knife (like a pad knife or a non-serrated butter knife) and a ruler. Basically you want to cut the paper, but only enough to fold it, not enough to actually cut it.
I also have had luck with a dull knife and a countertop separation when I can't find a ruler. My counter is made of two pieces and there is an indentation where the two pieces meet. I line up the cardstock on that line, and then use the knife to indent the paper into the indentation.
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Oh, and if you use the knife and ruler trick, you'll want to do it on a surface that will give slightly, like a wooden cutting board or a rubber mat, or even a cutting mat (which if you have would be handy because they will generally have ruler lines on them to help you center your score).
Dead ball point pen, edge of a credit card, the little chunk of plastic that doesn't always get punched out of the plastic milk bottle, a sharpened popsicle stick...run along a ruler or other strait edge.