You plant a message on an object within range. The message is verbalized when the trigger conditions are met. Choose an object that you see and that isn't being worn or carried by another creature. Then speak the message, which must be 25 words or less, though the speaking can take up to 10 minutes. Finally, determine the circumstances that will trigger the spell to deliver your message.
When these conditions are met, a magical mouth appears on the object and speaks the message in your voice and at the same volume you used when you cast the spell. If the object has a mouth or something that looks like a mouth (for example, the mouth of a statue), the magical mouth appears there, giving the illusion that the words are coming from the object's mouth.
When you cast this spell, you can decide whether the spell ends after the message is delivered or if it persists and repeats the message whenever the trigger condition occurs.
The triggering condition can be as general or as detailed as you like, though it must be based on visual or audible conditions that occur within 30 feet of the object. For example, you could instruct the mouth to speak when any creature moves within 30 feet of the object or when a silver bell rings within 30 feet of it.