Easy, just go to the top of the screen and place your pointer over where it says Personal, then click on Profile.
Then submit your username and password (its probably already filled in, and you just have to click on the Submit button).
Then scroll down to where it says Signiture and type in what ever saying you want to have at the bottom of all your posts. Be sure to click save, or update, or what ever it is at the very bottom of the page to submit it.
Voila! You're done.
1997 XLT SP 600
"Engage the Mechanism"