Buy a top that matches Light, L, or Misa's clothing. Light wears a white button-up shirt and tan suit jacket. L wears a long-sleeved white T-shirt. Misa wears various outfits, but they all involve a black, lacy top with spaghetti strap sleeves.
Purchase bottoms that match the character's clothing that you are cosplaying. Light wears tan suit pants that match his jacket. L wears dark, baggy jeans that are too long for him. Misa wears different skirts, but they are all black and lacy, some have a tutu shape. Misa also wears fishnet tights.
Wear shoes that match your character's. Light wears basic brown mens' dress shoes, Misa wears large, black, goth boots. L does not wear shoes. If you cannot go barefoot, sandals are a good compromise for L cosplayers.
Wear a wig that matches your character's hair. Light's hair is a short, light-brown men's haircut. Light has big, black hair that sticks up at every angle. Misa has blond hair with bangs, and she usually wears it with a black bow or a headband.
Style your wig like the character's haircut. Light's wig needs little styling, his hair is a simple mens' haircut. Misa's hair can be combed, curled at the ends, and pulled back into a large bow or left down with a headband. L's hair is large and poofy; hold it upside down and spray with hairspray to create a gravity-defying look.
Wear the appropriate makeup for your character when you cosplay. Light is a normal looking boy, and does not wear makeup. Misa is a pop star and wears makeup with pink or brightly colored lips, blush, eyeshadow, and long, black eyelashes.