Assigning a Prepayment Receivable Activity to a Receivable Prepayment Invoice

I am looking for guidance on how to create a Receivables Prepayment Invoice in the Form so that it can be applied to an Invoice (in the form); the process is entirely within Receivables, no AutoInvoice. Specifically, the set up steps tell you to create a Prepayment Transaction Type and a Prepayment Receivable Activity. What I cannot figure out is how to use the Prepayment Receivable Activity on the Prepayment Invoice, which I have assumed needs to be done rather than using Standard Lines.