Verified answer

That sounds like a hard costume to do...I would say a dark, long-haired wig, a nice fancy yet simple dress, and hoop earrings. By doing that and standing next to Justin it should be pretty clear.

jean skirt knee high striped socks long sleeve hooded shirt with some sort of a print really cool gloves with no fingers plastic beaded jewelry wear your hair down... if your hair isnt brown, get a wig very minimal natural makeup and a sheer semi gloss

if you dont already have long dark hair get a wig, wear colorful hoop earrings and i am pretty sure she has her own line of clothes so just pick and outfit from their.

Staples, knife, and Selena Gomez.

I think you can figure out the rest.