For a client who left owing. Firm, not rude.
Create the invoice for the balance.
Send it with a short message and the payment link.
Chase it after three days, then after a week.
If it is still unpaid after two weeks, tell me before the next booking is taken.
These are instructions for the assistant, carried out one at a time against your real records. Each one still goes through the same checks as if you had asked for it yourself.
Once you have an account, ask for "Unpaid balance" and it is added as your own recipe. From then on it runs on your records, not on an example.
From the first message to the chair.
Run it the same day. A no-show contacted that day usually rebooks.
The single highest-value habit in the business.
Two days after a first appointment, which is when they decide if you are their place.
Monthly. Everyone who has not been in for twice their usual interval.
A cancellation an hour before is money unless this runs immediately.