For this, you would need the version 0.0.34 of the Jerboa app. Now follow these steps.

• Install Jerboa v0.0.34
• Turn off your wifi and data connection or else the app would crash after trying to load anything from any Lemmy instances.
• Go to the log in page off the app.
• Give your Lemmy server address, username and password.
• Turn on your WiFi/data connection.
• Touch the log in button. You would be logged in to the app shortly and use the app.

If you wish, you can update the app. You will get a warning as the version is too low but you would still be logged in.