Finished a denim bomber from my two boys' old jeans and a few people asked how, so here's what I learned the hard way.
Deconstruct the jeans first, don't try to sew through layers. Cut the legs open along the seams, remove the pockets, and press everything flat. You get weirdly usable panels from around the knees and seat, and the worn spots give you that faded look for free. Cut all your patches before you sew anything. I pieced mine in a big chevron down the front and it only worked because I laid the whole thing out on the floor first and numbered the pieces on masking tape. Use a denim needle and go slow through the flat-felled seams. Skip the bulky crotch seam intersections entirely, just cut around them. Topstitching thread in a contrasting shade hides a lot of sloppy joins, and I Serged the raw patch edges so the inside doesn't shed all over the lining. For the barrel sleeves, cut the sleeve pieces wider than a normal sleeve and taper at the cuff, the bagginess is the whole point. Also, zip it up before you hem anything. The weight of all that denim changes the drape more than you'd expect.