How do I cut Semi-Rigid Aluminium Ducting?
Leather gloves and safety glasses.
Wide rubber band wrapped around the duct at the position you want to cut (a wide band stays in position better than a narrow one).  Leave a little extra to allow for smoothing out any irregularities at the end.
Permanent marker to mark the line you'll be cutting along.
Hacksaw to start the cut.  With the line marked, it's easier to keep the cut square, but you could also use a pair of tin snips to complete the cut.
File to smooth out the cut end and make it square if you've got any irregularities.