Ok, so say I want to create a vs lobby and play on 83.142.49.90:28012... what exactly do I have to do from start to finish?

1. open l4d2
2. open console
3. enter mm_dedicated_force_servers 83.142.49.90:28012 (connect commands are in the steam group annoucements and on http://sneakymonkeys.com/admin.txt)
4. Start lobby
5. Start game

I've now set the servers back to using search key again, so open a console in either l4d1 or l4d2 and use:

sv_search_key sneaky

then create a lobby and start the game as usual