Visit a forum that you have already registered a forum account on, and then click the "Login" link typically found in the top-right hand corner of the forum. You'll now be taken to the Sign In page. To login, type in your forum username along with your password on this page. After that is entered, click the "Continue" button found at the bottom of the page.
If you only have one account registered on the forum, you will be taken to the home page of the forum and will be automatically logged into that forum account. If you have multiple forum accounts, however, you'll be taken to a "Select Account" page. Once here, simply click on the name of the account you want to log in to the forum as. You will then be taken to the home page of the forum you're visiting and will be logged into the account you chose.
If the login attempt fails and you get an error message, it's possible the username/password you provided does not match our records. If you forgot your username or password, click on the "Forgot Your Password?" link found in the password box of the sign in page. On the "Recover Password" screen, type in either the username of your forum account, or your email address in the box provided. Once you click the "Recover" button, an email will be sent to your email account with further instructions on how to reset your account password.