The only complication with doing this with upcycled jeans is you have to find the spots where you can find a single piece that is long enough and wide enough for what you want.
If I had used pattern weights I could have moved my ruler around to cut all 4 sides without having to handle the actual denim as much.
Either do your cutting with an old blade that is reaching the end of it’s life cycle anyway or assign yourself a separate rotary cutter or pair of scissors for working with denim fabric – stretch or otherwise!
I applied this to both the stretch denim pieces and the fabric remnant I was using for the upper pannel of my bag. I think using this interfacing made a huge difference.
Denim needles are inexpensive and you’ll have the peace of mind that you won’t be breaking a needle in the middle of your sewing project.
Even though my old stretch jeans were relatively thin, not much thicker than a thick canvas cotton, I still put that tension up just to be on the safe side.
This is more of a finishing tip that is true of any bag making or sewing project – top stitching just always makes it look nicer.