Trying to play a left 4 dead server with more than 4 players

[Leo]

So i've been playing left 4 dead for a while and i noticed that some of the campaign servers have 18 player slots. I was looking for a tutorial on how to make this posible, but i couldn't understand the youtube steps or the forums, so i was unable to use the links until someone gave me a proxy web page to acces to them. After 3 consecutive days of failing, i created an account on the forums just to ask you if you can explain to me the steps as easy as posible, or at least give me a webpage that tells me how.

Sorry if i have any mistake, i'm not from the U.S.A