As soon as you open iMovie, a pop-up message should appear asking you to either open an existing project, quit, or create a new project. Click on the "Create Project" button to start a new project. A new window will open which is the iMovie workspace. If the pop-up does not appear and the first window that appears is the workspace, then you are working in an existing project. If it is not yours, go to the menu bar and select File > New Project.
∴Please note that this tutorial is designed for general editing on iMovie machines in ACD 403.