Jackets are tough cause your pits will bunch up if you bring in the arms at all. Whatever you end up doing, youll probably have to detach, and re-attach the sleeves. Get a sewing awl if you dont have one: Itll be really fucking tough otherwise. Heres how id suggest going about it.
1. Detach the sleeves.
2. Fold over, crease, and trim the shoulders. Youll want em just hanging over the edge of your shoulders, any further inward and the sleeves will fit like shit.
2b. Im not sure what exactly you mean by the traps width. Do you mean you want to alter the height, which would also bring the bottom up the jacket higher? If this is what you mean, grab the leather and fold it over, then lay an overcast stitch along the ridge.
3. Pin the top of your sleeve to the edge of the newly brought-in shoulder. Pin only to the top.
4. Stand in front of a mirror or have a friend mark how much you want the arms brought in.
5. Figure out how to bring in the arms, keep lines of stitches as straight as possible. For instance, dont fit it near the bottom, flare it at the elbows, and bring in at the biceps.
6. The arms probably wont fit the entire circumference of the hole they once were affixed to. You have two options, you can add an upside down triangle gusset at the gap, or draw a straight line from the bottom of the jacket to where the bottom of the arm should attach, and stitch it up. The triangle gusset will leave the extra room there, which may be nice to have, depending on your bodytype.
Pin everything well prior to sewing, since you are doing a lot of alterations. If you dont make the jacket skin tight, itll hide your potentially poor alterations, and you can still wear a hoodie underneath.