Sometimes a store-bought item of clothing does not fit as intended or desired. Some clothing alterations require the expertise of a tailor, but if a denim jacket's sleeves are too long, it is relatively easy to shorten them. With the proper tools and measurements, you can quickly shorten the sleeves of a denim jacket.
Measure from the cuff of the sleeve up to the point where you want the sleeve to rest on the wrist. Write this measurement down for future reference.
Use a seam ripper to pick out the thread binding the cuff to the sleeve.
Measure from the bottom of the sleeve (where the cuff was) to whatever point the sleeve needs to be, using the measurement from step 1. Mark this area with a piece of white chalk.
Cut the bottom of the sleeve off straight across the chalk mark. Insert a denim needle into the sewing machine.
Place the end of the sleeve into the cuff of the sleeve, just as it was before the cuff was removed. Sew the cuff in place all around the bottom of the sleeve.
Repeat the entire process with the other sleeve.
The sleeve may have to be tapered slightly in order to fit into the cuff once it has been shortened. To do this, simply fold over the fabric as much as you need at the seam and sew it in place.
Do not attempt this project without a denim needle. Any other needle will not be sturdy enough for the thick fabric and may break in the process of sewing.