01 - Combine powdered sugar, meringue powder, and warm water in a mixing bowl. Beat with an electric mixer on low speed until the icing reaches stiff peaks, approximately 5 minutes. If the consistency is too thick, add water a few drops at a time.
02 - Divide the royal icing into smaller bowls. Add a drop of vibrant food coloring gel to each bowl, mixing until evenly tinted.
03 - Using a small offset spatula or a piping bag fitted with a round tip, spread or pipe a white base layer of royal icing onto each Oreo. Let set for 10 to 15 minutes before proceeding.
04 - Pipe or paint colored icing onto the cookies to create classic sugar skull features such as eyes, nose, mouth, and ornamental flourishes. Use fine piping tips or toothpicks for detailed work.
05 - Enhance each cookie with edible markers for precision and sprinkles for accent if desired.
06 - Allow the decorated cookies to dry at room temperature for at least 2 hours, ensuring the royal icing is completely firm before serving or packaging.