Edible glitter can be brushed directly on fresh buttercream. The moisture on the buttercream helps it stick. You can also brush it on fondant or gum paste with the use of piping gel, edible glue, or alcohol. Glitter can also be used on chocolate exactly the same way. These days you can also find edible metallic spray in an aerosol can that can be easily sprayed over your decoration. The disadvantage of spray is that it can go everywhere!