Edible glitter and lustre powders are available from cake decorating stores, to apply dampen a soft bristled brush with clear alcohol and cover the surface of the icing. Then with another dry brush apply the glitter. Edible glitter goes a long way so start slowly as you can always add more but it’s very difficult to remove it. To apply the glitter dip a paintbrush in the glitter then holding the brush a distance above the cake, lightly tap the brush or shake to sprinkle the glitter evenly over your cake. If you just want a soft shimmer use a soft brush to apply lustre powder as soon as you have put the icing on, while it is still soft.