This isn't rocket science.

Do you have an old T-shirt that fits loosely? Or an old shirt of any description? That's a good start. Put it on and mark it with tailor's chalk for length, collar fit and shoulder width. Cut it along the seams and chalk marks. You will notice the back is a little longer than the front and you will want to add 4 or 5 inches to the bottom of the back (relative to the front length you choose) for kidney protection. There you have a pattern for the inside shell.

Green Pepper patterns at owfinc.com has more than one vest pattern.

A good general resource is:http://www.backpacking.net/makegear.html