My Dorodango process is a bit different than Gardner’s and that seen in the Nito Project. I use only pure clay—mined by the water extraction process—and I don’t add extra layers of clay after the initial forming of the mud ball.
The problem with this approach is that the clay can get hard in some sections of the ball while other sections are still wet and tending to crack. Even though I would check on it during the drying process and try to round it, it still wound up being mostly round but with some fairly lopsided proportions.
The answer to this problem is to catch it just before it is too dry to be changed but dry enough to withstand the use of a jar.
Get the Dorodango wet, and use the lip of a jar with a smooth lip to round it, gently. The small milk jars pictured below are excellent for this purpose!