To send a message, one must first find or create a rainbow with mist and then throw a golden drachma or anything with a good price into it, saying or thinking very deeply:
"Oh Iris, goddess of the Rainbow, please accept my offering."
"Oh Fleecy, do me a solid. Show (name and place of person)."
This is usually said aloud, though it works non-verbally as well. Then the caller says the name or location of the intended recipient. The way to end an Iris message is to wave one's hand through the mist, disconnecting the signal.
In The Battle of the Labyrinth, Percy receives a "collect" Iris message, asking him to deposit drachmas in the image for each five minutes of conversation. This happens only once and the sender was dead, so it is not clear whether this is a unique event.
Greeks use drachmas to activate Iris messages because this was the Ancient Greek currency. It is implied in The Son of Neptune that Roman heroes do not use Iris Messages (neither Frank nor Hazel had ever heard of this method of communication) since Iris is her Greek counterpart, and by then Romans used more advanced methods, like giant eagles.
Failed Iris Messages
In The Son of Neptune, Percy tries to contact Annabeth Chase via Iris Message, but the Iris Message can't get through. It is implied that this is because Percy can't clearly remember the person he is trying to reach as Hera stole his memory. It is also possible Hera herself had blocked the message.
|Percy, Grover, and Annabeth||Luke (though intended for Chiron)||Reporting about their quest.||The Lightning Thief|
|Percy and Annabeth||Chiron||Reporting about their quest and asking for advice.||The Sea of Monsters|
|Percy||Dionysus||So the camp can hear Luke's plans to reform Kronos.||The Sea of Monsters|
|Percy||Tyson||To talk.||The Titan's Curse|
|Percy||Sally Jackson||Telling her that he'll be coming home.||The Titan's Curse|
|Percy||Dionysus||Calling for help and to report about the Ophiotaurus.||The Titan's Curse|
|Bianca||Percy||Showing him Nico's current situation.||The Battle of the Labyrinth (twice)|
|Chiron||Typhon (his location)||To show the Head Counselors the status of the gods' battle with the giant.||The Last Olympian|
|Medea||Jane||To weaken Piper's confidence and reveal how her father was captured.||The Lost Hero|
|Thalia Grace||Jason Grace||To summon him to the Wolf House to free Hera.||The Lost Hero|
|Percy Jackson||None||Tried to call Annabeth, but it was blocked.||The Son of Neptune|
|Hazel Levesque||Reyna||Called her while she was taking a bath (unintendedly), Hazel warned her about the Giant Army.||The Son of Neptune|
|Gleeson Hedge||Mellie||To check in on her while he is with the seven.||The Mark of Athena and The House of Hades|