How do I call a disqualification?

Under Articles 102.13 and 102.21:

1. Raise one hand overhead immediately when you see the infraction. If you don't, there is no disqualification (unless the meet is run under World Aquatics procedures). The exception is dual-confirmation relay take-off judging, which is reported in writing without a hand signal.
2. Report it to the Referee on a signed slip giving the event, heat, lane and infraction.
3. The Referee decides. The Referee rules wherever judges' opinions differ, and can overrule on rule interpretation.
4. Notification: the Referee or a designated official makes every reasonable effort to tell the swimmer or coach the reason. Not telling them doesn't cancel the disqualification.

A disqualification can be made only by the official whose jurisdiction covers the infraction, so call only what you saw within your assigned area.