As other's have stated it's actual hard to mess up because after the cut you can still fine-tune it with the exception of actually cutting it too short of course.
I was paranoid about it all when I did it my first time too - a couple of weeks ago.
I used painter's tape as a guide of where to cut.
Use 32 or higher TPI blade. If your cut doesn't end up square and 90 degrees, file or sand by hand or carefully dremel sand paper bit (I don't know what it's actually called).
I'm sure you also already know this, but the 3mm gap between the top of the steerer tube and the top of the stem is critical. I had headset play no matter how much I tightened my top cap and realized that my gap was only about 1.5mm which gave no room to tighten up the headset.