Cheek-to-Cheek measuring tool

[nscrbug]

I just used the sit on a paper towel method of measuring sit bones. It works pretty well.

Ok, I'm sure it's explained in some other thread here...but could you spare me the hassle of trying to find it, and provide details on how this method works? Do you sit bare butt on the towel? Or in clothing? On a hard surface? I'm confused...