When the tournaments are announced, you can click the link to the website they have the sign-ups on. Follow the steps on the site to sign up.
The fnl tournaments are posted up here http://fnltour.challonge.com/. Simply click on one when there's one planned & sign up. There's also one on EUPC this week, you can sign up for it at the link below if you want to play. http://challonge.com/EUPCLEGENDS