Start your costume by deciding which version of The Joker you wish to become. Choose from the comic book incarnations, or from the animated Jokers that made appearances in Batman: The Animated Series and The Batman. Movie fans can choose between Heath Ledger's portrayal in the 2008 film, The Dark Knight, or Jack Nicholson's take on the role in the 1989 film, Batman. Once you've decided on your inspiration, piece together your costume. Use dark purple fabrics to create his trademark suit or use a straight-jacket to act out The Joker's frequent stays in Arkham. Comic fans can buy shorts, a straw hat, and a Hawaiian shirt to become Alan Moore's interpretation of The Joker in "The Killing Joke." Don't stick too closely to your source material, as personal touches on the costume will add to your interpretation of this constantly evolving villain.
Even more than the clothes he wears, it is the menacing smile and painted face that give the victims of The Joker nightmares. Begin with a white base. Your makeup should extend to below your neckline if you want to give the impression that your entire body is the same ghostly white color as your face. Apply red lipstick to your lips and decide if you would like to hold a smile for the entire time you're in costume or if you want to add a permanent smile with the lipstick. When attempting to emulate the smiles of Heath Ledger's Joker or Jack Nicholson's Joker, use face putty to create depth to your smile. Simply having lipstick smeared over your face will make your face funny for the wrong reasons.
Use green dye on your hair, but know that the canned hair spray will not fully cover darker hair colors. Instead use temporary hair dye bought in stores such as Target and Wal-Mart. These dyes will cover your entire head and still wash out after a few shampoos.
Take note of wearing makeup around your eyes. Combine purple into black makeup to give your eyes more of a bruised, sunken in look. This will add menace to your otherwise clownish face.
The final touches to your costume will be in the form of accessories. Singe the end of a deck of playing cards or an entire deck of Joker cards and keep them in your pocket at all times. Learn a few simple magic tricks such as squirting water from a rose on your shirt. Nobody needs to know the rose doesn't actually contain acid. Keep a plastic gun on you, and try to locate one that shoots out the "Blam!" flag. You should never be without a gag while in costume. For an added bonus, print out a picture of Barbara Gordon (Batgirl) and regale fellow party or convention goers with tales of crippling the hero in an effort to show her father what one bad day can do. If you have a willing female friend, suggest she accompany you in a Harley Quinn costume.