Try on the jeans and mark the desired finished length with chalk or pins
Measure the amount to remove from the bottom
Turn the jeans inside out
Cut off the excess fabric below the marked hem, leaving enough allowance for the original hem to be reattached
Unpick the original hem carefully if needed
Fold the jean leg up to the marked length
Place the original hem at the new bottom edge
Pin the original hem in place all the way around
Sew a straight stitch close to the original hem line to attach it
Trim any extra bulk from the inside if necessary
Press the hem flat with an iron
Repeat for the other leg, matching the first leg exactly