How to define an installment plan on customer invoices?

In order to manage installment plans related to an invoice, you should use payment terms in Odoo. They apply on both customer invoices and supplier bills.

Example, for a specific invoice:
Flexible instalment plan (120 days / 4 months):
10% immediate payment
20% after 30 days (Phase 1)
20% after 60days (Phase 2)
40$ after 90 days (Phase 3)
Balance after 120 Days

payment terms are not to be confused with a payment in several parts. If, for a specific order, you invoice the customer in two parts, that's not a payment term but an invoice policy.