There are few ways to cut depending on Carbon or Aluminium shafts.
I prefer a Hacksaw & Poles cutter.
First remove the grip with a Heat Gun, be patient and don't get too close as cork handles can get damaged by heat.
(Alternatively use hot water )
Clean the pole and measure required length.
Use Masking tape at the required cut length as this prevents fraying at the end.
Use some hot glue and slide handle back on.
Make sure you align it from the start to avoid and problems later.
If you are not confident ask at your local shop or practice on an old shaft!