Being able to cut paper neatly, and without slicing your hand open, is a pretty useful skill. It's not brain surgery, but there are a few things you need to know before wielding an X-Acto. Here is a step-by-step guide to help you step up your cutting game.
You need three things before you get started: A self healing cutting board, a good metal ruler with a cork bottom (this keeps the ruler from slipping), and an X-Acto with a fresh blade.
Put the mat on a flat surface.
Line up your ruler with the crop marks (Put crop marks on everything! It'll make it so much easier to cut things straight).
Now we're getting serious. Hold the ruler firmly with your non-cutting hand. You don't want to put too much pressure on it though. If you press too hard on the ruler, the ruler might slip, and you'll either mess up your straight line or slice off part of your finger. Don't put your fingers near the edge where you're cutting (obviously). Line your blade up with the ruler's edge at the top crop mark. Now you're ready to cut.
Always cut towards yourself, it helps you keep your hand steady. There's no need to rush either. Move the blade steadily, and slowly down the ruler. Depending on the thickness of the paper, you'll probably need to cut more than once. Don't move the ruler, just bring the blade back to the top of the crop mark.
Make sure you stop at the crop marks, and not at the edge of the paper. If you cut all the way to the edges you're going to lose half of your crop marks.
Rotate the paper and repeat, until you've cut all four sides.
Change your blade regularly. The sharper your blade, the cleaner your cuts.