One helpful feature inside of Jetpack Workflow is that from the projects list view, you can quickly complete a project.

To quickly close a project, under the Name column, you'll see that there is a bubble next to the name of each project. If you click on the bubble, the project will instantly be completed and moved to your completed projects area in the status filter. All the tasks in the project that you mark as complete will be marked as completed.

Once you mark a project as completed, you'll see a banner in the upper right hand corner that will fade away after a few seconds.

You can now find this project in your completed work and reopen if needed.