Easy, you just click on your profile then when it prompts the profile page, look at the left side of the screen (if your on your desktop/laptop) and under "Forum Profile Information" scroll down just near the bottom of the page you'll see there "Signature" , post your signature on that box and scroll down at the bottom and click on that small box that says "Change Profile" (downright bottom) and your good to go.
As far as I understand it, there are restrictions in length of the signature depending on your Status, e.g. Newbie, Jr. Member, Member etc.