In short, Snyder says he's spoken with Frank Miller who will be writing and drawing the graphic novel that the second film will be based on. Upon Miller's completion of the book, then, and only then, will they delve into writing a script for the movie.
Snyder continued and said he would not have any input into the novel and wants it to be a creation of Miller's 100%.
Snyder also said it will take place between the Battle of Thermopylae and the Battle of Plataea, the battle that narrator Dilios is at while re-telling the story of King Leonidas and the mighty Spartans to a new group of warriors at the end of 300.
As Dilios says, there's a whole year between Thermopylae and where he currently stands, in his final monologue he states,
There is a whole year between the battles to explore. And that is where we will find this film.
I would assume Dilios has a large part in it with hopefully David Wenham coming back to reprise his role.
Snyder also told IESB that would definitely direct this sequel/prequel and is looking forward to Miller's completion of the book.