The first step in frosting Christmas cookies is to make sure the cookies are completely cooled before you start decorating. Then, you'll need to gather your supplies - a piping bag or a ziplock bag with the corner cut off, a butter knife, and a variety of sprinkles and decorations. You can start by spreading a thin layer of frosting on each cookie using the butter knife. Then, you can use the piping bag or ziplock bag to pipe designs onto the cookies. Add sprinkles for extra flair!