Sergey Karjakin already qualified as he will be seeded as the challenger in the last World Championship match. (1 spot)
The top 2 finishers at the World Cup will be seeded in the Candidates Tournament. It will begin in a few weeks in Georgia. (2 spots)
The top 2 finishers of the Grand Prix will be seeded in the Candidates Tournament. The winner of Grand Prix No. 1 was Shakhriyar Mamedyarov. The winner of Grand Prix No. 2 was Alexander Grischuk. (2 spots)
The top 2 rated players (by rating average) will be seeded in the Candidates Tournament. The current leaders are Fabiano Caruana and Vladimir Kramnik with Wesley So a very close 3rd. (2 spots)
The final spot will go to the Organizer of the Candidates Tournament. The Organizer can nominate any player with a minimum rating of 2725 (on any rating list during 2017). Usually, this spot will go to the player from the country which hosts the event. (1 spot)
These 8 players will play a double round robin to determine the Challenger to Magnus Carlsen.