A collage contains a collection of images that were selected in the Collage Editor. A collage can only be created by clicking Done in the Collage Editor, which can be opened by clicking the Collage icon inside of the Tools dropdown in the Effects tab of the toolbar.
The images in the collage are initially arranged as they were in the Collage Creator. Click the Edit Collage button to reopen the Collage Editor for further editing. Clicking Done will apply the changes to the project.