(stolen from chiharu!!)
1 ★ Create a graphic (200 x 200 max size) to represent your personal "candy". It should have your username on it, but otherwise can feature whatever you want. Make it something special since it's self-representative.
2 ★ Make a post with the subject "trick or treat?". Put your "candy" somewhere in it, and be sure to repost these instructions.
3 ★ Then, go around other people's LJs and reply to them with either "trick" or "treat." If you reply with "trick," they will give you an LJ/DW/Twitter/tumblr dare that you have to perform before taking their candy. If you're too wimpy for that, simply say "treat" and take their candy. (See example here)
4 ★ List all your collected candies in your original "trick or treat?" post to show off your collection.
My eye candy: