As far as royal icing goes, this is my favorite. Grab a can of meringue powder at most any store carrying cake decorating supplies and you'll end up with a great batch of fluffy white stuff. I mention the Wilton brand only because I use it and it works well. I don't work for Wilton! However, I head over to their tent sale in Woodridge, IL every June to load up on super discounted baking supplies. Enter Tent Sale into the search box at Wilton online for details.
3T Wilton Meringue Powder
1lb powdered sugar (4 cups)
6T warm water
Beat all ingredients until icing forms peaks (7-10 minutes at low speed with heavy-duty mixer or 10-12 minutes at high speed with hand mixer). Makes 3 cups of icing.
Use this icing as the glue for your gingerbread house and accompanying decorations. It dries hard as a rock in just a few hours. Since it includes raw egg whites, don't eat this stuff. Nobody wants Salmonella for Christmas!
4 egg whites
4 cups powdered sugar
1t. lemon extract or lemon juice
Beat all ingredients at low speed until icing forms peaks, 7-10 minutes. Cover bowl with a wet towel while you work or (better yet) move directly into an airtight container. Use decorator bags and tips or a cookie press to pipe icing onto your gingerbread masterpiece.