Ahead or threaded?
If threaded, it couldn't be easier:
1) Measure the required length
2) Check your measurements!
3) When you're sure about the measurements, get an old headset and lock it so that the top of the uppermost nut lines up with where you want to cut.
4) Make very small cuts (using the uppermost nut as a guide) and keep rotating the steerer in the vice.
5) Pretty soon all your very small cuts should join up.
6) Keep cutting and rotating until you're all the way through.
7) Gently file off sharp burrs
Don't attempt to cut in one: your cuts will inevitably be guided by the threads of the steerer resulting in much cursing!
If Ahead, mark where you want to cut with the edge of some tape and proceed as above, taking greater care with your small cuts (you have tape as a guide not a steel nut!)
Good Luck: you probably won't even need it!