Flood icing is simply royal icing with water added to make a consistency to "flood" or fill in the outlined design. It's best to go slow, adding a little water at a time to get to a syrup-y consistency. The more cookies you decorate, the more you'll get a feel for it. If your icing ends up a little thin (like water), add sifted powdered sugar or some leftover icing from outlining to your flood icing.
Once the flood icing is "just right", cover it with a damp towel and let sit for several minutes. Using a rubber spatula (I love these!), run gently through the icing to break up the air bubbles that have risen to the top. Now pour into squeeze bottles. These can be found in the candy making section of any craft or bakery supply store. Walmart even carries them! You could also pour the flood icing into a pastry bag fitted with a tip. I used to do this before I discovered squeeze bottles...I wouldn't recommend it, though...it's a mess! :)
Onto the outlined cookies, squeeze the flood icing in a zigzag pattern. I usually do 3 -4 at a time. Don't worry, the icing won't harden by the time you get back to it and it gives the icing time to spread. Using a toothpick, spread the icing to cover the cookie.
This is how mine look, more or less, before spreading with a toothpick.