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.
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 citations equal to the number of players for that letter. In addition, when starting a letter, all the phantom entries must be written before any new entries for that letter can be created. You can cite a phantom entry that's already been cited before, however.
You cannot cite yourself. This also means you can't write a phantom entry which you have previously cited.
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. If a new player joins, all the letters including and after the current one can now hold one additional entry.
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.