Two Apple devices must both use Underpass in order to chat.
The two devices can be on the same Local Area Network (LAN), or they can both be on the internet. See Basic concepts for more details. To chat on a LAN, select "New Local Area Network (LAN) Chat". To chat on the internet, select "New Internet Chat".
When you select a new LAN or internet chat, you are presented with the chat settings. The first step is to enter a password.
The same password must be used on both devices. If you choose to leave the password blank, then the password must be left blank on the other device too. For security, it is highly recommended that you select a password for your chats. See Passwords and encryption for more details about passwords.
The next step is determined by the type of chat: