There are a couple of ways to go about this.
- You can paint on a design using fabric paint/pigment. The best bet would be to buy commercial quality pigment (aka something you have to order).
- You can sew on a full pattern over top of your denim. For jeans this would take a decent amount of fabric, time, and perfectionism, but would totally be worth it in the end.
1. find some fabric
2. lay a piece of paper over the denim where you want your fabric and trace a pattern.
3. tape/pin it to top of fabric and cut out your pattern. leave extra if your fabric frays so you can hem it or use no-fray.
4. pin the fabric on and sew your printed fabric on your denim.