I use something like a clamp or a scrap stem (the clamp end), which is what I'm using now as a guide.
If you go off a little from straight, clean it up with a file. You probably ought to clean up any hacksaw cut with a file anyway.

If the steerer is aluminum (unlikely) you can use a pipe cutter made for copper, but if it's steel, you need a bigger one designed for steel pipe.

If the steerer is CF reinforced plastic, you need to tape it and cut with a hacksaw, and don't compress it when you clamp it to cut.