So the first thing you need to do is download pidgin. You can find it here:
Download Pidgin Source
Now add a new account.
Change the protocol to XMPP.
Username is your league of legends log in name.
Domain is "pvp.net" without the ".
Resource is xiff.
Password is AIR_"your league of legends password" so if your league of legends password is 1234 its AIR_1234.
Connection security (don´t know how its called in english because my pidgin is installed in german) you have to set to "Force SSL (old-style)".
The Port need to be set at 5223.
Servers are Server: chat.na1.lol.riotgames.com (for NA) chat.eu.lol.riotgames.com (for EU West) Server: chat.eun1.lol.riotgames.com (for EU Nordic/East)
To this point all credits need to go to Jabe because all its written here he made up first and I got most of the information by him!