The first step in listening to Dipsea offline is by adding a story to Your Favorites. Not sure how to add a story to your Favorites? Just look for the heart icon next to a story anywhere in the app (example below on the listen screen).
Follow the steps below for more info on how to listen offline:
Make sure you are on WiFi (stories will only download when you are on WiFi)
Favorite any story you want to download by tapping the heart icon from either the story detail page or listen screen (screenshot below)
Go to the Your Stories tab >> Your Favorites playlist >> tap the three dots next to any story you'd like to download
Now when you are offline/on airplane mode, any of the stories you've downloaded while on WiFi will be available for listening!