I had to laugh when I saw this hack making the rounds again, because my nanny taught me this when I was little and it works every. single. time.
My nanny, which is what I call my grandma on my mom's side, taught us young.
The trick: you take the waist of your pants/shorts, MAKE SURE THEY ARE BUTTONED/ZIPPED, and wrap them around your neck. If the waistband fits like a necklace with about a one inch overlap, they will fit around your waist perfectly.
See the video below....