Using the basic instructions here:
How to sew a curtain using old jeans
The above link inspired me to sew a shower curtain or doorway curtain from old jeans. My tips, in addition to this article are:
*Cut a 10 x 10 inch square from an empty cereal box for a template. Use a rotary cutter on a cutting board, to cut your squares easily.
These new curtains are nice and thick for keeping cold air in or out of a room, depending on the season. So don't throw your ripped, tattered, and stained jeans in the garbage. Find fun and creative ways to make something new.