I need something squishy in between my butt and the hard surface, otherwise my butt is too squishy all by itself to make an impression.
A yoga mat works, or just a terry towel or blanket, folded in two or three, should. I used typing paper. Easier to make an impression on stiffer paper, plus it lasts longer.
The important things are (1) the underlying surface is hard and flat and (2) you're bent at the hip to approximate your riding position.
Once I made my butt-print, I circled the impressions on the paper (rather than a number, I had two ovals that aren't completely parallel), and took the paper saddle shopping with me. Easy to see which ones would support my sit bones and which ones wouldn't.
Speed comes from what you put behind you. - Judi Ketteler