4 lbs. powdered sugar
3/4 c. meringue powder
1 1/3-1 1/2 c. warm water
2-4 tbsp. oil-free extract or flavoring

Whisk together sugar and meringue powder. Ad flavoring to water and slowly incorporate to sugar while mixing.  The consistency will be like honey. Mix at medium speed until stiff peaks form, (about 7-10 minutes), stopping as soon as peaks form.  Do not use oil extracts, as the icing will not set up. Adjust a portion of the icing to a stiffer consistency, (like toothpaste), to use to outline the cookie or the different decorative sections.  This creates a border to ensure the icing does not flow over the edges. Use a thinner version, (like shampoo), of the same icing to fill in the outlined area.  Use a spray bottle to thin your icing. This is called FLOOD icing.  It will usually flow until it fills the entire cookie. Toothpicks can pop bubbles that may appear.

Great helps here: http://www.sweetsugarbelle.com/2011/07/outlining-and-filling-cookies-with-royal-icing/