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Okay, so you've got your cookies, you've got your flood icing. Now what?

It's time to choose your colours and fill your piping bags. There really is only one brand of food colour that is used in royal icing. Americolor seems to be the only product that doesn't alter the very delicate liquid balance you've just spent about 15mins adjusting. Trust me, I've tried other brands - liquid colour, gel colour - they just don't produce the same result. 
So choose your colours and decant about 200ml of icing into three small mixing bowls. I use ramekins. Add a few drops of colour to each bowl, reserve the icing left in the main bowl for your white supply. I try to stick to four colours per cookie project including white. But there's nothing to stop you from doing more if you have the inclination. 
Use a long handled teaspoon to mix the colour through the flood icing, ensuring every last scrap of white is combined. Americolour will dry darker, so with reds and blacks, don't drive yourself nuts trying to get full colour saturation in the wet stage. The colour will be right when it's dried. 
Set up your piping bags with piping tube sizes according to whether you are laying down a base colour or layering over the top. Flood icing is no good for pipe work on dry icing so you'll be using these colours for wet on wet techniques or laying down colours next to each other. I use a No. 3 tube for the base because it gives a thick layer, sets hard and gives the cookie a nice crunch. 
Fill the piping bag then seal it off. I use cable ties because they seem most effective. But you can also use kitchen string or rubber bands. 
Pipe a line around the edge of your cookie. Just do them one at a time. Try not to drag the piping tube through the icing. This needs to be a neat ridge. Hold the piping tube about an inch above the cookie at a 45 degree angle and guide the icing to fall into place. This is called a "dam". Fill in the space inside the dam straight away. You should see the icing connecting up with no "scars" visible. Every part of the cookie top should be filled. Pipe a little extra to get a nice level finish. Some people gently shake their cookies from side to side to help the icing heal. I try not to touch them as I invariably stick my fingers in the icing. Belle sits hers on a tray and shakes the tray. 
If you want to do a wet on wet technique, like polka dots, you have to drop the spots into the base layer before the icing can form a skin. This means you have to continue working on one cookie at a time. 
It's complicated, isn't it? Now you know why cookie artists charge so much for a single cookie. 
Once you have iced all your cookies set them aside to dry for 8-12 hours. That's right! Last minute rush jobs are not possible with sugar cookies. They really do need that amount of time to dry, and putting a fan on them is advised. 
Can I suggest, if you're really into this and you want to know more, check out the youtube channels of Sweetambs, Montreal Confections and Haniela. They are such inspirations and can teach you so much more than me. And of course you should check out my dear friend Belleissimo Cookies' youtube videos too. Happy cookie-ing!