Setting up reminders
Up to three email reminders per booking, and at most one of them can also text.
Reminders are set once for the salon under Settings → Reminders (opens in a new tab), not per booking. You choose how far ahead of the appointment each one goes.
You can set up to three reminder times — a week ahead, the day before, and a couple of hours before is a common shape. Clients get emailed reminders unless they turn them off themselves.
Text
Exactly one of your reminder times can also send a text. That's a limit on purpose, not a missing feature.
A client can only receive one reminder text per booking regardless of how many rules you set, because opting out happens at the number rather than per message. So there's no useful second text to schedule — pick the one lead time that does the most work, usually the day before.
Email reminders are on by default; texts are off until the client opts in and gives a mobile number. That applies to clients you imported too — a number in a spreadsheet isn't consent.
One reminder, once
Reminders are sent by a job that runs continually through the day, and each one is claimed before it's sent, so a client doesn't get the same reminder twice if the job overlaps itself.
Email and text are tracked separately for the same appointment, which means one can fail without taking the other down with it.
If they're not arriving
For texts specifically, there's one cause that accounts for most cases and can't be fixed from your side — read why your client isn't getting texts.