First, you need to install 123FlashChat, downloaded from 123FlashChat.

Find the chat server in /usr/local/123flashchat. Settings are in directory /usr/local/123flashchat/server/etc. To make the chat server start at boot time, copy /usr/local/123flashchat/server/fcserver.sh to /etc/init.d and run

update-rc.d fcserver.sh defaults 

as root.