Each round, each of the players writes an entry on a subject beginning with that letter of the alphabet, which must cite three other entries, one already written, and two not yet written. Citing an unwritten entry creates a phantom entry. In the first round, you don't have to cite a previously written entry, obviously. You can cite a phantom entry that's already been cited before by another player, or yourself, however.
After a round starts, a player may call dibs on a single phantom entry. This means that no other player may write that phantom entry, although obviously the player who called dibs should be considered to be under serious obligations to write it before the round is over.
There may not be more entries per letter, phantom or otherwise, than the number of players. So you can't cite a new unwritten entry if there have already been a number of phantom entries cited in that letter equal to the number of players. In addition, when starting a letter, all the phantom entries must be either written or dibbed before any new entries for that letter can be created. If a player has cited all the currently existing phantom entries for this letter, they may create a new entry, as they can't write any of those phantom entries.
You cannot cite yourself. This also means you can't write a phantom entry which you have previously cited.
Players can enter the game at any time, by noting that they are doing so on the Discussion page, or just adding themselves to the Players page, and possibly by writing an entry as well. If a new player joins, all the letters including and after the current one can now hold one additional entry. If a player drops out, and there are more phantom entries for a letter than there are players, each person writes at least one, and then whoever gets to the remaining ones first can write them. Provided they aren't breaking the "Don't cite yourself" rule.
A player cannot cite a phantom entry if it would mean that every available entry space on that letter would be cited by them, because then they won't be able to write an entry when that letter comes around. In the hopefully unlikely event this means they can't cite any phantom entries existing, they can just cite an additional previously written entry, then. Also, at the letter Y you can swap one phantom entry citation for a previously written entry citation, and at Z, you can swap both of them.
You can cite extra previously written entries, or phantom entries, if you really want and they're relevant to the subject at hand, but don't create more than two new phantom entries with any given entry.