Law 15 - Throw-in

When the whole of the ball passes over a touch-line, either on the ground or in the air, it shall be thrown in from the point where it crossed the line, in any direction, by a player of the team opposite to that of the player who last touched it. The thrower at the moment of delivering the ball must face the field of play and part of each foot shall be either on the touch-line or on the ground outside the touch-line. The thrower shall use both hands and shall deliver the ball from behind and over his head. The ball shall be in play immediately it enters the field of play, but the thrower shall not again play the ball until it has been touched or played by another player. A goal shall not be scored direct from a throw-in.

Punishment:

If the ball is improperly thrown in the throw-in shall be taken by a player of the opposing team.
If the thrower plays the ball a second time before it has been touched or played by another player, an IFK shall be taken by a player of the opposing team from the place where the infringement occurred, subject to the overriding conditions imposed in Law 13.

Decisions of the International F.A. Board

If a player taking a throw-in plays the ball a second time by handling it within the field of play before it has been touched or played by another player, the referee shall award a direct free-kick.
A player taking a throw-in must face the field of play with some part of his body.
If, when a throw-in is being taken, any of the opposing players dance about or gesticulate in a way calculated to distract or impede the thrower, it shall be deemed ungentlemanly conduct, for which the offender(s) shall be cautioned.
A throw-in taken from any position other than the point where the ball passed over the touch-line shall be considered to have been improperly thrown in.