To create a movie, you first create a movie project.
Important: iMovie saves automatically as you work on a movie project; you don’t have to save as you go.
In Projects view, click Create New, then click Movie.
If you’re editing a project, click the Projects back button on the left side of the toolbar (shown below), and then click Create New.
If you have more than one iMovie library open, click the Library pop-up menu and choose the library where you want the movie to reside.
After creating your project, you add clips to it from events in the Libraries list and your Photos library. When you return to Projects view, you’re asked to enter a name for the project.
The resolution and frame rate of your movie project are determined by the first clip you add to the timeline. iMovie now supports 4K video at 30 frames per second (fps) and frame rates higher than 30 fps for 1080p and 720p video. If you want your project to be set to 4K resolution, the first clip you add to the timeline must be a 4K clip. If you want your project to be set to 60 fps, the first 1080p or 720p clip you add to the timeline must be a 60 fps clip.