First, you need to open a conversation with Ava.
Let's say that you tell your assistant to "schedule a meeting with John next week at Starbucks".
Just to make sure that it understood, it will respond with:
Juliano, you asked me to schedule a meeting with John at Starbucks someday between Monday, November 21 and Friday, November 25 from 9am to 5pm.
It will then ask you "Is this correct?" and if you reply with "Yes", it will then tell you that it will email John, introduce itself as your assistant, and ask them to choose a time-slot based on your availability.
It will let you know once John accepts, changes their mind, or rejects your meeting.
You can always ask your assistant for some "training on meetings" or "hints".