You can buy a pattern (both Simplicity and McCall's have them in their costume section).
You can make a duct tape wrap to make a pattern. It requires a friend to help you out.
Put on a t-shirt you...
What you need is tin foil string then some sticks .1. you tie the sticks together like a raft with the string then when you get it the size and shape you want then cover the entire thing with tin...
they're are home remedies to making prosthetics. but it'd be best to buy the actual things needed. for most prosthetics you need to use things like spirit gum, liquid latex etc. There is youtube...