Have a love for apples.Eat them all the time. Say, "It's worth going to the human world for these apples." or something along the lines. Pretend to go through withdrawal symptoms if you can't eat apples.
Copy his personality.Ryuk is creepy and doesn't play for any sides.
Ryuk wears a lot of metal like his earring.Wear some chains and a heart attached to a chain on your left ear.
Carry around a Death Note wherever you go.It pulls the whole thing together.
Follow around a guy who acts like Light.Talk to him and tell him how to work the Death Note.
Wear a lot of black.Find a way to make it look like your shirt is sewn into your skin.