I hope you mean you use a craft knife, and not just the razor, this is difficult even with the knife, especially cutting the mats, it's impossible with just a razor to get a clean edge, with no blood!
I like a beveled edge, but that's something that I never learned how to do right, and that's why I buy the precut. Here's a picture of a craft knife.
Craft Razor Knife and Cutting Mat Set - SK3225