Buyers can leave a review after a project has been delivered. Click My Orders, and then click into the project.
If the order is open, click "ACCEPT" on the delivery message, or "Mark Order Complete" on the message box. This will open the review option.
If the order has been completed, click My Orders, then click into the project, and scroll to the review option at the bottom.
After the project has been completed, Click Manage Sales, then click into the project, and scroll to the review option at the bottom (see image above).