I think it’s really satisfactory to take an old piece of denim and embellish that with some colourful patches of fabric and thread.
Doing some hand embroidery is a great start if you have never done anything like this before.
You can start with one spot and wear your piece in between and later on extend your work.
For example: Cut out an interesting shape from a piece of printed fabric. You can use a normal glue stick to adhere it to your jacket. From another fabric remnant, cut out some more shapes. Choose matching thread. In this case I used thin knitting cotton, but you can use embroidery floss or a thicker machine thread as well.
Here is a great link to embroidery stitches you can use.
Make sure your patches are all stitched on properly , so you will be able to wash your jacket as usual.
The glue will disappear in the laundry.
Here is an example of a work in progress of one of my students.
A good place to start is on the back of a jacket.
Or on a skirt.
I hope this will inspire you and I would love to see what you will make!