Test Closure is a document which gives the information about the testing status before completing the testing process.
Once we got the sign off from project and customer team, we need to collect all the testing deliverable (such as Test plan, Test conditions and Test results) and stored it in identified project closure locations. Later we need to mention the path of the location in test closure document.
We need to identify all the open and closed defects and document it.
We need to collect the data's related to test schedules and its variance and deviations, testing metrics (Productivity, Defects, etc.).
We need to collect all the lesson learned including risks and mitigation actions.
All the above need to be documented using one template.Template would be vary based on the organization.