Payment plans are typically used for high priced classes like a retreat, semester, or teacher training.
After creating a class with an optional payment plan, your customers will see an option on the class's checkout page to use the payment plan. They'll also get to preview the schedule they'll be charged.
Notice how:
The payment schedule is listed for the customer: ie. $50 now, $50 in 15 days, $50 in 30 days, $50 in 45 days
There is a checkbox for the customer to use the payment plan option (its not required)
When creating the class, on the payments page, toggle on this 'Allow customers to use a payment plan). Then click 'Add installment'.
Set a schedule that mimics the one below (where the 'days after purchase' increase with each installment).
You can also edit existing scheduled payment plans from Customers > Payment plans.