We mix the layer mash with buttermilk, milk, or plain yogurt w/active cultures. We've also used home-canned (organic salt-free) creamed corn (winter) or tomatoes (warm weather) pureed in the blender in a pinch.
You don't want the mash too wet, so just add a small bit at a time. You just want the mixture to hold together so it gets crumbley. In the winter, we warm the liquid a bit before mixing.
regards,
keljonma
if edited, probably for typos....