You are correct - that was the wrong way to do it.

A prepayment is for when something is paid [or invoiced] in one go, but needs to be charged to the profit and loss account gradually. For example, insurance or business rates.

What you've done is pay someone in advance of their invoice(s) being received - that's a payment in advance rather than a prepayment. (I've never considered it before, but it's plain to see now that the term 'prepayment' could be potentially confusing in this context!)

The easiest fix is two simple steps:

1) Reverse the bank payment you've made to the prepayments account by posting a bank receipt to that account. (I assume you've done it as a bank payment - though describing it as a debit and credit as you have makes me wonder if you've done it as a journal; no matter, though, the result is the same).

2) Post a supplier payment to the purchase ledger account for the designer - the same account that you will post the invoices to.

In step 2, if you've put any of the invoices on the system already, you can allocate them against the payment when you put it on, but you presumably won't have the full amount in invoices yet - the difference will be saved as a payment on account. Alternatively, just post the whole payment as a payment on account without allocating.

In future (or now, for those invoices you have if you posted the whole amount as a payment on account) you can go to supplier payments, and allocate any invoices on the account against that payment on account. When doing this, always ensure that you are allocating 'from' the payment matches the amount you are allocating 'to' the invoice - otherwise you might accidentally post additional payments.