I use warm water to dilute the washable Elmers glue in a spray bottle, spray it over the batting then take my hand and smooth over what I have sprayed before I spread the quilt top or the backing on it.
I do sandwich it before I start then turn half of the backing/quilt top back and spray half at a time. If I'm going to start quilting on it right away I press it lightly just to dry it quickly, and if I'm not I just leave it alone for a few hours until it dries before starting to quilt. I've had very good results doing it this way and it is never stiff or never has "hard spots". Good luck!
As for how much water to use in diluting the glue, I dilute it just enough so it will spray a fine mist.