OK, a quick guide:
From the main site, select the "Play Now" tab.
Then, select either "Play real-time" or "Play turn-based."
Real-time games require all players to stay at the computer during the course of the game*
Turn-based games allow for people to take a turn, go do something else, come back, and take another turn.
Which one is chosen affects how the game is launched.
For both types, though, choose a game, select create table (the "play" triangle, if it is one of your games, or the "create a new table" button if it is not one of your games), and set some options. These options include how much time is allowed per turn, minimum/maximum number of players, skill level, and so on. Additionally, if there are variants to the game, these are set here as well.
If the game is created real-time, then wait until players have joined. After the minimum number of players have joined, the "launch game" button will become active. A message will pop up explaining the timer, and some other warnings. When everyone has clicked the "I agree" button on the message, the game starts.
If a game is created turn-based, the pop-up message should occur when a player joins the table, or is assumed to have been agreed to by the table's creator if they do not click the create table link explicitly, but just let the timer run out. The game will then start when either the maximum number of players has been reached, or 24 hours after table creation if the minimum number of players has been reached. If 24 hours pass and the minimum has not been reached, the game is abandoned.
If you have further questions, feel free to ask.
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*(OK, for those technical nit-pickers, there are ways around that. For a newbie, can we just say that for the most part, this is the way it is? Thanks.
)