Set them up
Settings → Notifications → Client reminders. Turn reminders on and choose the timing: 24 hours before, 2 hours before, or both.
Which is best
The 24-hour reminder gives people time to reschedule; the 2-hour one catches those who forget on the day. Many businesses use both.
Who gets them
Customers with a contact method on their booking, including appointments you added yourself. Someone with no email address and no mobile number cannot be reminded.
Checking they went out
Every message is listed under Settings → Notifications → Delivery History, and on the appointment and client timelines.