(please excuse the picture quality, I should probably buy a camera)
cut off the arms, not right on the seam. Give yourself some extra fabric for sewing onto the new jacket body.
If theres an inside liner, which there will most likely be, use a seam ripper to just cut that out and discard. you don't need extra layers to be sewing.
So you have an arm.
turn the arm and jean vest inside out and pin the two together.
You may be able to sew the two together with your sewing machine, but I have a very old one so I just hand-sewed it. Just make small loops going through both layers.
This is the end result. It's kind of hard to see, I'll post a better picture soon. There are different ways to do this DIY but the way I chose was so that the arm looks as though it's coming out from under the vest. Almost like you're wearing a jacket under a vest. I left the vest edge very ripped and destroyed looking. If you want more of a polished look, I recommend turning the jacket body inside out, leaving the arm right side out and slipping it in the jacket body then sewing the two together with a sewing machine. When you turn them right side out, there will be no viewable edges. Just like this tutorial here. Hope that makes sense! Aaaaaand one more idea I thought could look cool is taking a jacket (like say an army style jacket) and giving it the look of leather arms by dyeing just the arms black!
Super easy with a great effect! like this v
Go make some jackets!