This can be done on the game page:
http://www.longturn.org/game/LT39/
Be sure to login and check that you have a green mark on the "pc" column. For confirming participation you need to click the confirm button on the bottom right of the web page. Do this before the game is about to start. If you have changed your mind and don't want to play, please cancel your confirmation. That can be done on the same page. This will help us to keep the game with as few idlers as possible. No shame in cancelling participation if you have no interest or time to play
Another reminder about the actual LT39. The game is not started automatically but it's done by hand. Once the signups and confirmations are closed the admins will add the players to the players list and then make the final tests for the game. After that the game will be started a bit later. I there are delays a post will be made on the forum. The first turn will be extended 3*23h so that everyone will have a chance to play the first turn.
For LT39 a native client is needed. That means Freeciv 2.5.x client. Also, the latest Qt client allows you to see if someone is sharing vision to someone. No more that easy deceptions when you are sharing vision to an ally
If you have any questions don't hesitate to ask.