Ref the cutting direction, I agree about the cutting direction advice. I find if the line of the cut is in line with my fingers, hand and elbow, so I am pulling the blade in the same direction as my forearm, I get a clean and controlable cut. Try to push/pull the blade sideways, even by a few degrees, and the cut goes off-line. This means turning the card with the other hand to keep the cut direction constant. Takes practise, but it works.
Tim P