You use a royal icing like Antonia\s royal icing recipe. Take part of it and slowly add a few drops of water at a time. Mix with beaters. Keep checking it so it will flow off of the knife/spatula. I burp it to get rid of air bubbles and put in plastic bottles or bags with tips appropriate to size and fill in the areas. I let each area dry at least 6 hours ( this wet icing really needs to dry so you don't smug it.) Then work with next area to fill.
Or are you thinking of a poured icing like ganache or sugar icing used for petit fours?
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