1) removing or cutting off the existing grip,
2) cutting down the length with a tubing cutter (for steel shafts) or a small hacksaw (for graphite shafts),
3) re-installing the grip,
and most important: 4. adding weight to the clubhead in some manner to restore the swingweight feel of the club.
If clubs are shortened and no weight is added back to the clubhead, the chances of the length reduction having the desired effect are somewhere between slim and none.