First, the usual disclaimer; measure twice (or more) before you cut as there is no going back. I like to leave the steerer long enough to require a 5 mm or 10 mm spacer above the stem as it assures the steerer is fully within the stem (important for a carbon steerer, less so for steel) and gives you the option of raising the bars slightly in the future.

Once you have determined where to cut, you can use a radiator clamp around the steerer as a cutting guide by positioning one edge directly on the cut line and using it to guide the saw. Use a fine tooth hacksaw blade (32tpi is good) and let just the weight of the saw make the cut. For a carbon steerer, I like to put a wrap of masking tape around the steerer spanning the cut line to reduce the chance of splintering the carbon. Also, work the cut around the circumference as you go rather than cutting straight across. Once the cut is complete, smooth off and bevel the edges with a fine file or sandpaper.

Do not use a pipe or tubing cutter on a steel or alloy steerer and certainly never on a carbon steerer.