Recording prepayments is a vital process and I'd be happy to show you how you can easily do that in QuickBooks.
Among other great features which QuickBooks offers, recording prepayments is one of them. You can do this by following the steps outline in this article here: Record a retainer or deposit. I encourage you to go through the steps and in case you get stuck, don't hesitate to reach back here.
Let me know if you have other questions. I'm here to help.