For Harley Quinn: You can either do Harley's most well-known look in the movie, or wear a glittery dress similar to the one above. Just make sure your hair is dyed platinum blond with Harley's signature blue and pink tips.
For the Joker: In addition to the green hair and white face makeup, you can get a red button-down shirt and pair it with a silver tie and blazer. Or go shirtless and throw on a long, sparkly purple jacket and blue jeans.