When two characters play each other at an individual sport, both players should make dice rolls using their Athletics (including specialism in the relevant sport) + CO.
The character with the higher dice roll result wins the point or game (depending on the sport).
Depending on the sport, there may be a bonus (e.g. +2) to the player in possession or serving. Possession or serving typically either alternates between the characters or is retained by the winner of the last point or game.
If a game lasts for a long time, the referee should require characters to make Stamina dice rolls (to avoid becoming tired and to keep going when tired).
When a character plays as part of a team, all members of the team make dice rolls using their Athletics (including specialism in the relevant sport) + CO.
The team's dice roll result is then calculated as the average of the best dice roll result in the team and the worst dice roll result in the team. The team gets a bonus on +2 on the dice roll if they have regularly played together, and a +1 bonus on the dice roll if their morale is good.
As for individual sports, there may be modifiers for possession or serving, and the referee may require all team members to make stamina dice rolls in a long game.