Leagues last for roughly a month. You may enter any number of times but only one deck at a time.
You play five matches per run. You can join the league at any time.
To find a game sign up and then create a game in Constructed, Specialty, Freeform Tournament Practice with "Penny Dreadful League" as the comment.
Top 8 finishers on each month's league leaderboard win credit with MTGO Traders.
When you complete a five match league run for the first time ever you will get 1 tik credit with MTGO Traders (at the end of the month).