The dry ground is a pumice based chunk of material made to be applied to paper. The purpose of such a ground is to give a bit of "tooth" to a surface which will grab the pastel and make it smooth for layering.
They are available in Black and white, Sepia and Buff, Umber and Grey
You may tint the white or black DG with any pastel color and disperse it on the surface by just working it into the ground.
To apply the ground, place your paper on a flat surface and rub the ground on the paper. Then rub in the ground with your hands, I suggest wearing a glove, otherwise you will have no hand left after doing this repeatedly! Once the surface is prepared just draw into it with any pastel.