The example here is a great jacket, but the sleeves were too full. Slenderizing them gives the jacket a whole new look!
This is basic sewing stuff, and I am a basic sewer. You can do it yourself, it's easy!
You will need:
Scissors (fabric scissors are best)
Sewing Machine (you CAN do it all by hand, but it will obviously take longer)
Fabric Crayon, or other marking tool
Step 1: Determine the correct fit
Put on your jacket and pinch one of the sleeves at the seam (starting from your armpit to your wrist). You will use pins to mark where you will be sewing. Be sure to not pinch around the arms too tight, or you will have trouble bending your elbow. The most critical part seems to be on the forearm a few inches down from your elbow... determine the maximum amount of fabric you will be taking in at this point.
You should do this with only one sleeve for now. You can do the other one later.
Step 2: Detach the sleeve lining
If your sleeve doesn't have a lining, skip this step.
If it does, cut the threads that bind the lining to the jacket sleeve. In this example, this step was easy; the lining was easily detached from the top hem.
Peel back the lining to expose the jacket's original seam.
Step 3: Mark your seam
Where the original seam meets the armpit, taper your marking towards this seam in order to blend the new one with the old.