How does the installment payment plan work?

It's the same two-step structure as your registration and program fee.

You pay the registration fee upfront to reserve your spot, then pay the program fee within the window described in “How and when do I pay for my Group Trip?” in the Payments & Deadlines section of the Help Center. If you have credits from a previous refund, they're automatically applied to reduce what's charged to your card at checkout.