Set them up
Settings → Notifications. Turn reminders on and choose how far ahead they go out — a day before, a couple of hours before, or both.
Which is best
A reminder the day before gives people time to reschedule; one a few hours before catches those who forget on the day. Many businesses send both.
Who gets them
Every customer with a contact method on their booking, including appointments you added yourself.
Checking they went out
Every message sent is listed under Settings → Notifications → History, and on the appointment and client timelines.