Laws of the Game
At the moment of delivering the ball, the thrower must:
stand facing the field of play
have part of each foot on the touchline or on the ground outside the touchline
return the ball into the game from the point where it left the field of play
All opponents must stand at least 2 cm (0.02 yds) from the point on the touchline where the throw-in is to be taken.
The ball is in play when it enters the field of play. If the ball touches the ground before entering, the throw-in is retaken by the same team from the same position. If the throw-in is not taken correctly, it is retaken by the opposing team.
If a player, while correctly taking a throw-in, deliberately throws the ball at an opponent in order to play the ball again but not in a careless or a reckless manner or using excessive force, the referee allows play to continue.
The thrower must not touch the ball again until it has touched another player.