I'm sure there are a few Instructables out there that cover breaking down jeans and many more with ways to use them.
I am writing this -able as a prelude, of sorts, and reference for Instructables-to-be which use the pieces as cut here... the only piece I am unsure of what to do with is the fly. Maybe it will come to me :)
Step 1: Supplies
jeans which have been deemed "unwearable"
measuring tape -or- ruler
Step 2: Fillet of Backside
Cut to the thin side of simple seam, going through the hem, a couple inches up the leg.
From the inside, release simple seam at bottom hem, and cut across bottom hem a couple of inches.
Proceed to cut, following the simple seam, up the side of the leg; across the yoke, below the seam; back down leg, against flat felled seam (double stitched); and across hem.
Measure down 1 in. from bottom point of back pocket.
Level with the top of the pocket, cut across panel, separating pocket from leg.
Repeat with back of second leg.
Step 3: Carpenter Cuts
Start at the inseam, cutting through the hem, up the leg/center back seam, across the yoke and down to loop.
Cut through loop first, then continue down the seam.
Lift loop and cut as before, 1 in. from bottom point of pocket.
Start as with loop-side, at the inseam. Cut up the inseam, across the yoke and down to where the side seam intersects the pocket seaming.
Cut side seam free at top edge of pocket seam.
Move down just under the pocket and cut through the seam, then around the edge of the pocket, next to the seaming, stopping where you would cut across, 1 in. below point of pocket.
Pick up here with cutting across the panel, separating the pocket from the rest of the fabric.
Step 4: Fillet of Front
Using the seam ripper, release the belt loops attached to the front panels of your jeans.
Starting, again, at the simple seam, cut up the leg, being mindful of where the front pocket is.
At the bottom of the pocket, where it is sewn into the seam, cut through the seam.
Move up to where the yoke connects to the front panel. On the yoke side of the seam, cut to waistband between yoke and side seam.
Cut across the front panel, just under the waistband.
If you encounter rivets, pull the edge of the waistband as far back as you can, then cut between the waistband and rivet. Do the same when you get to the zipper.
Slide your scissors between the front panel and the seam of the fly. Cut along the fly, down the center seam, back down the leg, and across the bottom hem, close to the seam.
Once the panel is free, pull the pocket to the side; measure 1" down from bottom of stitching; cut across panel, minding pocket; open panel to the outside; cut, on panel side, up the seam, leaving seam attached to pocket.
Repeat with second leg.
Step 5: Zipper
Use your seam ripper to release tacking on the left side of the fly.
At the top of either side of the zipper tape, pull tape from fly to expose stitching and use your ripper to release zipper tape. Usually, once you get it started, with a little muscle, you can pull most of the tape free, then use the ripper for the few remaining stitches at the bottom.
Repeat with other side of zipper tape.
There is a thin strip of denim backing on the front side of the fly. Once you have ripped the tape free, you can just pull the backing off.
When the zipper is out, pick the little bits of broken thread from the tape--you don't want thread getting in the works and mucking up functionality.
Be mindful of the zipper head position... sometimes you will have stops at the tops of the tape, but more frequently, there are no stops and, while it isn't impossible to slide the head back onto the teeth, it can be troublesome.
Step 6: Waistband & Yoke
Using the ripper, release the belt loops (and tag, should there be one) from the yoke, and cut across the top of the yoke, close to the waistband.
Cut out yoke panels, close to the seams.
Step 7: Seams
Cut the two bits of inseam from either side of the center seam.
Cut the two bottom hems from the outer seams.
And there you have it, all the bobs and bits. Stay tuned if you have scrap denim :)