Download the measuring app for IOS
Along the back, measure the distance between both ends of the shoulder bones (medical name: Acromion)
Measure the circumference of the neck. Measure just below Adam's apple.
Measure the distance from just under the bone behind the ear to the acromion (end of the shoulder bone). Make sure to keep your head upright.
What is your (predominant) sleep position and mattress firmness?