All the steps here must be done with the jeans inside out!
I made mine with small jeans and that's why the front looks like a inverted V, if your jeans are bigger you can get it to close like a regular jacket! :D

The jacket can be as long as you wish, just mesure the length of your arms from the shoulder down + 2cms (for sewing) to make the sleeves with the legs of the jeans.
You can also play with the length of the sleeves 3/4 or... well, you get the idea!

If you want the sleeves to be tighter do like with the red line, if they are skinny jeans you probably want to leave them as they are.
To help the jacket look better in the shoulders you should make a little cut (yellow line)
Do the same in the other leg mirrored (the shoulder side should be the longer one)

Make the holes for the arms by un-stitching in the sides!
I made the un-stitch in mine a little up (see the main image).
Remember the button will be your neck, so if they jeans are too small you might want to un-stitch a bit lower (to have the widest part of the hips matching the widest part of the shoulders).
Othewise you can put the sleeves as up or down as you want.
**You might have to un-stitch all at the end to make it fit, but I made the holes first to help me check they were at the same height.

un-stitch the inner legs and the crotch part (like if you were gonna turn them into a skirt)
And take the zipper off without cutting the fabric

Sew the sleeves to the holes made on the step 4.

sew the back of the jeans (what used to be the bum) in a strigth line.
If you follow the shape it has you'll have a small bump in your back. :D

when you are happy with the fit in the back you can check the fit under the armpit.
If it's not fitting, un-stitch, mark the fabric as desired and sew.
Now it should be fitting good!

Hem! hem! hem!

Add the buttons like in picture 9.
and!

TaDaa!