How do I signal foul play?

If the match organiser has given assistant referees authority to signal foul play, you signal that foul play or misconduct has been seen by holding the flag horizontally and pointing infield at right angles to the touchline (Law 6.27 and the match official signals).

Then:

1. Stay in touch and carry out all your other duties until the next stoppage in play.
2. At the invitation of the referee, enter the playing area and report the offence to the referee. The referee takes appropriate action.
3. If your verbal report leads to a player being sent off, submit a written report to the referee as soon as possible after the match.