Never Never NEVER 'hem' a kilt! the pleats won't hang properly and people like me will publically mock you...

first step: make sure that you can properly tighten the straps and buckles, and move them so you can. I can often shorten a kilt by a couple of inches with this one step.

2nd step: move the straps and buckles down by the required amount (they will stay at the same point on your torso when you tighten them, which lifts the kilt.

3rd step: shorten from the top if required (bearing in mind that kilts in general and military kilts in particular are worn higher than modern eyes find usual.