1. Choose File → New.
  2. In the New Presentation task pane, click the link From existing presentation.
  3. Select an existing presentation and click Create New.

The design template(s) and existing slides in the original presentation are loaded as the starting point for the new presentation. Because the new presentation is unnamed, the first time you save the new presentation, you will be prompted to provide a name for the file.

The major difference between starting a presentation using a design template versus from and existing presentation is that a design template starts the presentation with no slides--only the slide-title master pairs are used, whereas starting with an existing presentation uses the slides from the source presentation as a starting point for the new presentation.