Hi Patty, here's a lovely video that will show you how to reupholster a chair or couch - https://www.youtube.com/watch?v=MS7AaUaoogU. You would do the same using your jeans but would need to sew them all together first to have enough fabric to cover the chair
This tutorial may be useful https://www.hometalk.com/6119955/how-to-make-blue-jean-bar-stools
What type of chair? Can you post a picture?
Have an idea of the chair project do you intend to do I’ll give us an idea of what advice to offer.
Dining chairs are probably easiest chairs to tackle. Upholstered chairs need more yardage and skill.
You can rip jeans apart at the seams and then quilt the size piece of fabric needed to cover various areas of your chair.
First de-construct your Jeans and form a fabric with the parts. Then make a paper pattern of your chair parts including a seam allowance . Pin the pattern and cut your Jeans fabric . Then tack the pieces together and try it on the chair for fit. Once satisfied all is well, go ahead and machine sew. You will need to secure the bottom edges with Staples or Tacks and maybe use a trim to cover them. Good luck!
here's an idea from hometalk!
For any project involving sewing denim, I would recommend that you use a quality brand Jeans needle, and a Hump Jumper, or a Jean-a-ma-Jig.
This blog does amazing things with blue jeans, I am sure you can find a project here. https://www.pillarboxblue.com/denim-3/