This should work for any amputation at knee or above. Not sure there's enough pant leg left for lower amputations.
Warning - This project requires sewing with a machine or by hand. Sewing uses sharp pointy things like pins, needles, and scissors. The writer is not responsible for any damage you do to yourself or your customer or your customer's pants.
Step 1: Test Pin for Fit
For my customer, we already had all his sweats pinned at his preferred length, so I just went with that. If your customer hasn't done that, pin the pants and have them wear them around for a couple of days to make sure they are right. Folded pant legs do weird things when in a wheelchair and transferring.
MAKE SURE YOU ARE WORKING ON THE CORRECT LEG! (Yes, I did the wrong leg the first time. He had pinned them himself and was wearing them backwards. Sweatpants do weird things when there's only one leg in them.)