1) An easy way is to type your message in the box. Scroll down. Click where it says name/url. Type your character's name in the box and post
2) Another easy way is to type your message in the box, and scroll down. Click where it says anonymous. You can sign your character's name in the post itself, or just stay in voice.
3) Also easy: post as yourself, but sign your character's name.
4) Or, sign-up for an e-mail account under your character's name. This is more work, but satisfying because your character's name is right there above the post.
So, there you go! Now, let's get started. Come in character!
