To copy a slide from one presentation to another, first, open up the presentation you want to copy the slides into. Next, click on Add Slide and select Copy From Presentation to view a list of all of your presentations. Now, select the presentation you would like to copy the slide from. All of the slides in the deck will appear. Choose the slide(s) you would like to add, - use Shift + Click to select multiple slides - then, click on Add Selected Slides to copy them over.