Take an old long sleeve shirt and lay it out flat in front of you.
Take some chalk and trace an breast plate pattern to the front of the shirt.
Take a piece of newspaper and trace half of your pattern onto it. Fold over and cut out.
Pin to the other piece of fabric or old shirt- preferably a good contrasting color. Pin down the pattern and cut out, allowing a half inch seem allowance.
Pin a hem all around the garment and sew the hem.
Once you breast plate is complete, pin down and sew to the first shirt.
Keep the right side top (it will be on your left when it is worn) portion unsewn so you have enough room to get your head comfortably through the collar.