You don't have to do anything special to use Transcribe offline. You just need to be logged-in when you have an internet connection. Transcribe will automatically configure itself to work offline when you login. Later, when you need to access the app when you're offline, you just need to visit the app link: https://transcribe.wreally.com/transcriptions. You might want to bookmark it.
Note that only Self Transcription (minus dictation) works offline. Automatic Transcription requires an internet connection.