First get a board and put it down where you will be doing the cutting to protect the table. I also use a utility knife to cut with, the secret is to use a solid steel ruler held down tight and slide your blade along the steel ruler, or a piece of straight steel , maybe 1/8 of an inch thick or thicker, or use a square, same thing.
You could also use some kind of clamps to hold down the ruler , material and the board all at the same time.
The extra time and effort to gather up these simple things I have mentioned makes it So Much Easier and much more accurate cutting too along with less potential for cutting your own precious fingers.