Before jumping into creating a custom layout, let's spend a minute examining layouts and placeholders.
Layouts specify the overall arrangement of slide content, while placeholders contain the individual pieces that you can arrange to your liking. All slides have a default layout, but you can override it with another. Although you can customize existing layouts by rearranging, adding, and deleting placeholders, since you'll create similar tropical vacation slides for different destinations, you'll save time by creating a custom layout instead.
Next, you'll see how to go to Slide Master view, as that's where you customize layouts.