*note* If making icing ahead of time, this is a good consistency to store it at, and you can thin it when needed.
To thin the icing to piping consistency, use a spray bottle to mist water into your icing and stir. Continue until you get the desired consistency. You can also use this tip if your stored icing starts to get a crust on it. Mist it with water & it should refresh it.
Piping consistency should be about the thickness of toothpaste. It need to be soft enough to flow smoothly through the tube and curve without breakage, but stiff enough as not to lose its shape. It should hold its shape, but not keep a peak any longer. Your icing is ready to fill a piping bag.
*note* To prevent the tip from crusting, keep your filled piping bag in a glass with a damp paper towel at the bottom.
To thin your icing to flood consistency, use the spray bottle method as before until the icing is the consistency of shower gel. *note* if you will let the thinned icing sit covered with a damp towel for several minutes, all of the air bubbles will rise to the top, and you can easily stir them out.
Put flood consistency icing in a bottle for decorating.
Repeat these steps for every color you plan on using for your cookie design.