In order to watch Crunchyroll offline, you'll first need to upgrade to the Mega Fan or Ultimate Fan subscription tier and download the latest version of the Crunchyroll app. From the episode screen, select the downward arrow button to download. Then, navigate to "My Lists" and choose "Offline."
Note that only select titles are available for download and will expire after 7 days unplayed, or 48 hours from when you begin playback.