After many attempts at cutting art card size paper for my artwork, I have found that using a paper cutter is the most consistant way. First, of course, buy a paper cutter. I found one at a local discount hardware store for $12.00. I taped a strip of card stock on the 3 1/2" vertical line and one on the 2 1/2" vertical line, each to use as guides. Once they are in place, you can butt the paper's edge up to the cardboard or cardstock that you taped on the 3 1/2" line or the 2 1/2" line...whichever you prefer first. the cut the paper all the way down. Then turn the paper and do the same thing on the other side. From there you can repeat the process until you have as many cards as you can cut from one sheet. Here are some pictures below to demonstrate the process.

Strip on 3 1/2" line.                          Strip on 2 1/2" line.                       Cutting paper for 3 1/2" cut.             Cutting paper for 2 1/2" cut.