Directions:

1. Decide which colours you want to use for decorating, and divide the royal icing into as many bowls. Mix in your food colouring until you've achieved the colours you like.

2. Fill your piping bags with the coloured royal icing.

3. To flood your cookie, first make an outline of the area you want filled in. Firmly squeeze the top of the piping bag and use your other hand to direct the tip, making a starting point by pressing down then lifting the tip up, pulling the icing in the direction you want it to go.

4. Once you've finishing outlining the area you want to fill, begin filling in the area with icing.

5. When you've filled the entire surface with icing and only a few tiny gaps remain, set your piping bag aside. Hold the cookie gently on either side and shake it until no gaps remain and the icing is smooth. Tap the cookie a few times on the table to ensure the surface is smooth. Repeat all the steps to fill in any other areas that need to be coloured.

6. If adding any decorations to your cookies, such as the buttons, eyes, and nose on this snowman cookie, do so quickly, while the icing is still wet (it will begin to dry almost immediately). For the snowman's eyes, we simply used a toothpick dipped in black royal icing to add them on.

7. Set aside your decorated cookie and allow it to dry for several hours or overnight.

For more royal icing tutorials, check out these links:
Outlining 
Feathering
Marbling