All you need is a ruler and something that has a small, hard rounded tip like a rub-on plastic tool or my personal favorite which is the tool that came with the Fiskars Shape boss to press down paper into shapes. As long as the tip is at least 2 mm wide (otherwise the paper can get damaged)and rounded anything is fine.
Then just put the ruler where you want the crease and slide the tool firmly (press pretty hard) a couple of times up and down the line. I use my cutter mat underneath since the pressing down could leave a mark on your table otherwise. Then remove ruler and fold the paper along the sunk in crease. There, perfectly folded card without any lines, tears, crinkles or damaged paper!