[Bongholio]

Ganjastar
go to the first post it tells you there in the white area.
C:\Program Files\Steam\steamapps\common\left 4 dead 2\left4dead2\cfg
For a listen server make an autoexec.cfg and copy/edit the commands
(such as l4d2_witch_speed 5.0) there and save it.
For ordinary dedicated server I guess you make or edit server.cfg
and upload it to the host server.