Cutting guide and new 24 tpi hacksaw blade (32tpi for carbon, not carbon specific blade as I always assumed until put right by largest bike manufacturer in world).
Pipe cutters designed for softer metal used in plumbing, not aluminium alloys or cromoly steel used in metal steerer tubes 😉
Trick is to twist the saw frame clockwise into the cutting guide whilst cutting to run the blade hard against the guide plate, angle the tool at 45 degrees and soft pressure whilst cutting, should ensure a clean, square cut.
Finish outside and inside diameter with semi-round file for steel/aluminium or wet grit paper for carbon