1. Cut a piece of string 18"/45.5cm 2. Hold the top to your C7 vertebrae and drop the bottom end. 3. Now feel all the way down to where string ends. It will fall either above or below your Iliac Center (essentially your love handles and where you rest your hands when you are putting your hands on your hips). PS: If the string goes past your love handles, that means your torso is shorter than 18" = medium size pack. If the string ends before your love handles, that means your torso longer than 18" = large size pack.