Granny and eggs etc., but before you cut thread on a spare nut - it'll help re-form the ends of the threads after cutting, though it'll still need filing clean after.
EDIT - a couple of nuts would, if locked against each other, provide an adequate one-sided guide of course.
I've never used a guide, but I've only ever cut unthreaded steerers. Many folk swear by a heavy duty jubilee clip as a guide, and a fine bladed hacksaw. A pipe cutter would, I fear, tend to follow the thread. A good sized vice could also be used as a cutting guide.
