What sort of soil do you need to do this? I am struggling to imagine doing this with the sandy silt loam that surrounded me as a child [1].
[1] http://www.landis.org.uk/services/soilsguide/series.cfm?sern...
[1] http://www.landis.org.uk/services/soilsguide/series.cfm?sern...
Make very loose mud with water in a bucket, let it settle for a minute, pour off whatever is floating on the top, keep whatever hasnt settled to the bottom, let it settle for a few hours, recover the settled portion. Repeat if desired for more refinement. Wrap in cloth and hang for several hours to reduce water content.
You are keeping particles which settle to the bottom after one minute and before one hour (or whatever, the minimum and maximum times are adjusted to suit needs)
I have one on my shelf here that I dug out of a river bank and made with my kids... we saved it and I shined it up by sprinkling sifted clay dust onto it and rolling it on a hard surface as it dried