Notice what is bothering you
A sleeve can feel wrong because it is too long, too narrow or placed differently from the sleeves you usually wear. Identify the irritation before planning an alteration. Does the cuff cover your hand when you reach for something? Does the fabric pull at the elbow? Does the whole jacket lift when you move? Shortening the cuff will not solve every one of those problems.
Try the jacket over its intended underlayer. A shirt cuff may be part of the look, or you may prefer it hidden. A thicker knit can change how the sleeve travels along your arm. Fasten the jacket if that is how you wear it, then stand with your arms relaxed rather than bent for a photograph.
Check ordinary movements
Reach forward as though using a keyboard, bend your elbow and put a hand in a pocket. Sleeves naturally move during these actions. The question is whether they settle back comfortably and whether the amount of exposed wrist suits you. A little movement is different from a cuff that needs constant pulling.
Look at both arms. Posture and the way a jacket sits can make a small difference between sides, so a professional fitting is more useful than copying a single measurement from an unrelated jacket. If you wear a watch, try it too. A narrow cuff catching on the watch may need a different solution from a long sleeve.
Understand the cuff before changing it
Buttons, vents, zips and decorative stitching can affect an alteration. Ask how the detail will be handled and whether the result will retain the proportions you like. A plain cuff offers different possibilities from a sleeve with functional buttonholes. Do not begin by cutting away fabric or removing trim in the hope that the rest will be obvious.
For an informal jacket, a deliberate rolled sleeve may be all you want. Test whether the fabric stays in place without becoming bulky. For a tailored piece, pinning during an appointment lets you compare lengths before committing. At collection, repeat the movements that originally bothered you. The successful change should make the jacket easier to wear, not simply produce a tidier still image.
