Once your project has been marked as complete by your service provider you will receive a project sign off contract which you can accept or decline. The process is the same as accepting or declining a project.
First, make sure you are happy that the project is complete and the work has been done by your service provider to your satisfaction, then click the Review and Sign button. A window then opens where you can apply your signature if you accept. If you do not believe the work is complete, click the Decline Contract button and provide a reason.