To measure yourself, you will need a measuring tape, and a friend. Our size chart has the W-S length terminology in it, which means waist to shoulder length. In essence we are measuring vertical coverage.
Measure from your shoulder to your hipbone.
This measurement should give you your W-S length, then you can compare it to the size chart.
If you are in between sizes it is usually best to size down. Keep in mind you don’t want the product to wedge between your neck if you crane your head back (Freeriders flipping, or Racers tucking).