If you'd like to create a custom order for a buyer, you'll need to make sure you have an upgraded PRO Seller account (click here for details on how to upgrade).
Your buyer will then need to contact you via your inbox requesting a custom order. Once they have done that, you'll see in your inbox the option to 'Create a custom order'. This allows you to describe the service you'll be providing, set a price and provide the buyer with a link to make payment for the order. A new order page will then be generated where you can liaise with the buyer in the usual way.