From the 2011 U.S.S.F. Advice to Referees on the Laws of the Game
(http://resources.ussoccer.com/referee/Books/Advice to Referees.zip):
3.4 SUBSTITUTION PROCEDURE
After the player being replaced has left the field, the referee must signal permission for the substitute to
enter. A substitution is not complete and the substitute may not take part in the game until he or she
has entered the field of play. Referees who deviate from the formal process by which a substitute
becomes a player—whether in the interest of saving time or because the steps are thought to be too
complex and cumbersome—do so at their own peril and will eventually discover that the Laws of the
Game specify the procedure for very good reasons. Deviations may lead to situations that the referee
cannot settle within the Law. For example, a substitute might commit a violent act after being allowed
to enter the field before the player being replaced has left. Then the referee must decide whether the act
was misconduct and, if the guilty party is dismissed, whether the team plays short. Regardless of other
deviations from the correct substitution procedure which the referee may allow, a substitute becomes a
player for purposes of determining the consequences of misconduct when the substitute enters the field
after being beckoned by the referee. When the substitute has become a player as a consequence of
meeting this requirement, the player being replaced ceases to be a player.
Upon receiving permission from the referee, a player leaving the field is normally expected to exit from
the closest point on the perimeter of the field. Referees should not permit exiting players to waste time
by unnecessarily selecting a more distant exit point. Substitutes entering the field with the permission
of the referee must do so at the halfway line.
Note: The Laws of the Game do not require that a player agree to be substituted even when the
substitution has been requested by a team official or team captain. If the player refuses to leave the
field, the substitution request must be denied.
The referee must whistle to restart play after a substitution.