The hardened icing is a thinned down Royal Icing. When it is thin enough, you can pipe a border, let it set for a few minutes, then flood fill. Making Royal Icing requires all the utensils and mixing bowls to be grease free so do a good hot soapy water clean before making it.
Here is the Wilton Royal Icing Recipe:
Ingredients:
3 tablespoons Meringue Powder (Joann's or Michaels has this in the baking section.)
4 cups confectioners' sugar sifted (do not need to sift if you use right from a new bag. Once the bag is opened and has been stored, you need to sift.)
6 tablespoons warm water.
Makes: Icing serves about 3 cups. Add your color to small amounts of icing AFTER you make all of the icing so that you will have plenty of white icing for all the colors you need. A little icing goes a long way.
instructions
1. Beat all ingredients until icing forms a peak (7-10 minutes at low speed with a heavy-duty mixer, 10-12 minutes at high speed with a hand-held mixer). Very important to mix the entire time even though it seems long.
NOTE: Keep all utensils completely grease-free for proper icing consistency.
*When using large countertop mixer or for stiffer icing, use 1 tablespoon less water.
You can use plastic icing bags (Joann's or Michaels) or a ziploc with a hole to make application a little easier and less messy.