Dry cookie dough can be extremely difficult to work with, as it may crumble apart in your hands rather than forming the perfect balls or slices you wish to make. If you don't remoisten it correctly, the situation may not improve even after you bake the dough. Your final cookies can be dry and hard rather than delicious and chewy. Fortunately, this is not the kind of difficulty that requires you to throw out your dough and start again. You can simply moisten the cookie dough to your liking to create perfect cookies.
Add approximately 1/2 tsp. of liquid to your batch of cookie dough. Use a liquid that is already in your recipe, such as milk, water or egg whites. If you are feeling adventurous — and precise — you can combine two or more of the liquids in your recipe to avoid adding too much of one flavor. For example, you could put a drop of vanilla extract into your measuring spoon and then fill it the rest of the way with milk.
Mix the liquid into the dry cookie dough. Be careful, though, to not mix more than absolutely necessary. Over-mixing your dough can be another cause of hard, dry, crumbly cookies, so doing this will undermine your attempts to make softer cookies by adding more moisture.
Add another 1/2 tsp. of liquid to the dough if it is still too dry. Again, mix only as much as absolutely necessary to blend the liquid with the dough. Repeat these steps until your cookie dough reaches the perfect consistency. Then continue baking the cookies as your recipe instructs.