Altering the shoulder of a jacket requires some basic sewing skills and proper measurements to ensure that the alterations meet your needs. When preparing to alter the jacket, measure the distance between the shoulder's edge and the neck. Then, make sure that your adjustments line up with those measurements. If you take too much off the shoulder, you will end up with a scrunched up shoulder. Taking too little off the shoulder makes the shoulders look droopy, rather than form fitting.
Turn the jacket inside-out so that you can modify the inside of the jacket without causing visible damage to the outside portion of the jacket.
Remove the shoulder seam using the hooked end of the seam ripper. Using the hook on the seam ripper pull up on each individual stitch to cut the seam. Do not completely remove the entire seam, just cut around the shoulder to the area where the armpit falls. Remove the shoulder pads if your jacket has them.
Fold the outside of the sleeve over to bring the shoulder in. In general about one inch should be enough for most people. If you need to, fold it over an additional inch to meet your needs. To get an idea for how much you need to alter the shoulder, place pins in the shoulder to keep it intact and carefully put the jacket on.
Restitch the shoulder from the inside of the jacket using your sewing machine. When finished cut any loose fabric from the inside away from the jacket. Cut it close to the stitch to avoid any discomfort while wearing the jacket.