Click any button labeled "Reply", "Post" or "Sign". Replying to a message, posting a message and signing a Guestbook or Message Board are the same thing. Note: You do not have to register or sign in to post on a Guestbook or Message Board.
You'll then go to the "Post a Message" page. Simply type in your identity, fill in the subject header, type your message, and then click "Post Message." You may also include your email address if you'd like. To share your homepage with others on Message Boards, simply type http:// and the rest of the URL in the text of your message.