To schedule a custom payment plan for a customer, click Customers > Payment plans > Add new Scheduled payment

Next, choose to associate the scheduled payment with a specific class or membership purchase (or not, via none) > select the amount, then the date the charge will hit.

Note: the customer should have a card on file to ensure this charge goes through.

After creating the payment plan, you can always edit the date/amount or cancel it via the dropdown menu to the right.