This article explains how to create a simple slideshow of your favorite pictures in Google Photos. Information applies to Google Photos in a web browser. The Google Photos apps for iOS and Android do not currently offer slideshow functionality.
Here's how to create a simple Google Photos slideshow.
In Google Photos, select Albums from the sidebar and select the album containing the photos you want to display in a slideshow.
Select the photos you want to display in the sideshow. Continue making selections until you have selected all the photos you want to display in the slideshow. When choosing photos for a slideshow, you can only select photos in one album.