T-shirt (this will get cut up), Scissors, material vest will be made from 2x2 meters should work, fabric glue (Goop), Velcro, not the sticky kind, and a sharpie marker.
So to start the easiest way to make a template is to find a shirt that is the correct size, but that can get cut up.
Cut along the side of the shirt where the seems of the armpit meet. cut all the way down to the bottom of the shirt. Next cut half the sleeve off, this helps later.
Now that you have made your template, stretch the shirt onto the fabric, be sure you are drawing on the side that no one will see and try to keep the lines as straight as possible. You may want to safety pin the shirt down to improve accuracy. Draw around the template, and draw an inch extra so you can fold it back to give the vest a clean sewn look.
Be sure to not cut it down to much as it can be adjusted later and cut down.
Once cut out it should look something like this
Don't forget to cut into the fake sew zone every few inches
Take a belt lying around and put the vest on, overlapping the under armpit sides. from this adjustments may be needed depending on how it fits.
Once everything is fitting good, glue the one inch sides back to the hidden side of the vest. (not the best pic