With a standing mixer with a whisk attachment (or a hand mixer), whisk powdered sugar, meringue powder, water (starting with 6 Tbsp of water), and vanilla extract.
As the mixture begins to come together, add in orange juice. Whip on medium speed until the icing holds stiff peaks.**
Divide the icing into separate bowls and tint with gel food coloring (if you only want to use white royal icing, you can omit this step). This is the piping consistency for royal icing.
For flooding, thin the royal icing with water, beginning with a teaspoon at a time until it’s at the desired consistency (it should be thick enough to hold its shape, yet thin enough it drizzles off of a spoon).
When decorating, pipe a border around the edge of your cookies. Flood the centers of the cookies, then use a toothpick to spread the icing to the edges. Allow the icing to set before adding more decorating details (if you are planning on using the wet on wet technique, you can read in the post for more tips).
Allow the cookies to air dry (about 4 hours) before stacking on top of each other.