This method only works when you have to shorten the sleeve by ¾ inch or less.
If you need to shorten the sleeves by more than ¾ inch, there are two other methods i hope to post some time later.
Carefully unpick the cuffs using a seam ripper.
If there are pleats, pin the pleats and stitch to hold in place. If they have gathers, then gather by hand.
Mark the cutting line. Remember to allow some seam allowance.
Cut away the amount you need to.
Carefully sandwich the sleeve between the two layers of the cuff.
Pin and baste before topstitching. Make sure that you ‘catch’ all three layers when you stitch.