Made the tiara out of royal icing and it turned out great, but want to add some sparkle/pizzazz to it. I tried dusting it with disco dust but it won't stick. Also tried lustre dust which does not seem to want to stick. Any suggestions are appreciated. Thanks

Lightly brush the surface with either edible glue or water even egg white not too much then sprinkle on the sparkles and they will stick to it. hope this helps x

after piping the royal icing while it's still wet add the disc dust. It sticks really well. Once dry lift it up and let the excess fall off

The easiest way:
Put some vodka in a mister bottle. Place the tiara on parchment or paper towel. Mist the tiara and then immediately sprinkle with the disco dust. The vodka will make the dust adhere. Because vodka evaporates quickly the tiara won't be wet or sticky. You'll also be able to save the extra dust that fell on the parchment!! HTH