If you see a foul, look at me, and wait a heartbeat or two. If I am blowing the whistle (or bringing it up to my mouth) or if I am calling "Play-on", you don't need to do anything. If I am not doing these things then I did not see the foul, or from my position on the field I did not see it as a foul.
You now have to make a decision on whether you think play should be stopped. If you decide to allow play to continue you don't have to do anything. If you want to call out "Play on!" and/or give a signal with your off-field hand go ahead. If you think play should be stopped, stop, raise your flag and give it a good shake. If I decide to stop play, I will blow my whistle. Immediately, point the direction of the free kick with your flag parallel to the ground. If it is a penalty kick, drop your flag and run to the corner. If it is an IFK, raise you non-flag hand to so indicate, and jog to the corner if it is in the penalty area for the attacking team.
If I decide not to stop play (note: I am not overruling the AR. What a strange concept, as if by magic I can decide something I did not see was or was not a foul), I will give the advantage signal ("Play on!" and upswing of the arms) to indicate a foul has been committed (and observed by a member of the officiating crew) and that I am allowing play to continue. As I bring my arms down I will clearly wave you back to your position.
If I miss your signal (ouch). Wait a few seconds, drop your flag and return to your proper position. If the foul directly results in a goal let me know (I have already discussed this with the AR). If not whack me upside the head at half-time or after the match.
If an off the ball foul occurs behind my back, I always want you to signal it and I want the other AR to mirror the signal. These fouls are the most dangerous to the referee's control of the match and I always want to deal with the players involved.
Remember, selling any call depends on presence. The further away the foul the harder it is to sell the call and the more likely that I will not see your signal. Keep that in mind when fouls are not within 10-20 yards of you.
Have Fun!!