If you have wet clay from using it on a pottery wheel or because you got it too wet, then you first try to drain out any water from the clay. Sometimes if you squish it the water comes out so that it's more usable.
 Once it dries, you can use it to remove excess moisture from clay so that you can use it over and over.
 How dry is it? If it's really dry (like at the leather stage where it's about the consistency of hard cheese) then you're going to need to approach it differently as apposed to something that's mildly dry.;
, If your clay is mildly dry, just work some water into it by kneading it like dough. Usually that makes any semi-dried clay that you used before into nice usable clay.
 If your clay is really bad and leather hard, then you should coat it with water all over and cover it in a very thoroughly soaked towel or paper towel. Let the clay sit there for a bit before you try to work some water into it.
Remove excess moisture or water.
Make a large slab of plaster.
Feel the clay.
Knead water into mildly dry clay.
Dampen and let extremely dry clay sit.
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