The signal for the corner kick occurs when the ball passes over the goal line last touched by a defender. The Assistant Referee needs to be in position to make the call and must be on the goal line. Once the ball passes over the goal line last touched by a defender, the Assistant Referee signals as shown below. The Assistant Referee should stand with their feet together and hold the flag at a 45 degree angle pointing towards the ground.
Note: If the ball passes over the goal line and then returns quickly, the Assistant Referee raises the flag straight up (this gets the referees attention) and once the referee blows the whistle, then the Assistant Referee signals for the corner kick.