What kind of mixture should I use to make my own back-yard mud-pit?

I want to create the kind of mud-pit rich people soak in when they visit a spa in Yurp.

I'm going to dig a 1-meter deep pit (That's 3-ish Freedom™ Feet) and fill it with some combination of soil and volcanic ash. I thought for sure there would be some DIY instructions for such a project on the internet .... but I'm not finding one and I don't know where else to ask.