Sometimes (not often) the tournament creator will post a message here or on another board announcing the tournament. If they do not do that, the best way it to just periodically check the "Tournaments" page found on the left of this page. Many tournaments have all games included, and many others are a single game, or a few games. You might have to check through them to find the one(s) you want.

However, being a brain pawn (non-paying member) you are limited to one tournament entry at a time, and cannot join another tournament until either your section completes and you do not move on to the next round, or the tournament finishes :-)