Making a Shell
Making a shell out of an existing shirt is quite easy to do. For this particular tutorial, I used a jersey knit fabric. To accomodate for the type of fabric used, you may have to adjust the tension of the thread on your sewing machine.
Take your fabric and fold it in half with right sides together. If you happen to be using a fabric with a pattern, try to match up the pattern on the edges so that it will appear continuous when your garment is complete. Simply use a shirt that you like the fit of as your pattern to cut out the fabric you'll need for the shell.
With a sewing machine sew the sides, leaving spaces for the arms, and tops of the shoulders, leaving the neckline open.
Then using an iron create a crease for the hem, neckline and sleeves.
Sew the ironed parts down to create clean lines and trim the excess fabric from the hem. Now you have a shell, perfect for wearing with jeans.