On the Site
You can add a story to a reading list by opening the story and selecting the 'Add to Reading List' button on the left side of the screen. You can then add it to a preexisting reading list or creating a new reading list to add it to.
In the App
You can add a story to your Offline Library list by selecting the story thumbnail, clicking the "Read Now" button, and pressing the download icon in the bottom left corner. You will then be able to access the book under the Offline Library tab.