Tuck your chin and find your C7 vertebrae (the bump that sticks out at the base of your neck).
Place your hands at the top of your iliac crest (the top of your hips on either side) so that your thumbs wrap around your back. If done correctly, it will look like you're about to give someone a scolding.
Using a flexible ruler or measuring tape, measure the distance between your C7 and the imaginary line between your thumbs. This is your torso length.