I was watching videos on YouTube and i saw people playing custom campaigns on left 4 dead. How do they make those and download them?
To make your own campaigns in L4D you have to install the L4D SDK.
Luckily, if you bought L4D, it’s free to download, and get started on map making.
1) Open steam and select the ‘Tools’ tab.
2) Look for "Left 4 Dead Authoring Tools".
3) Install it.
Once it installs, launch it and a small screen with the old school Steam skin will pop up, to begin making maps open up "Hammer World Editor" to learn how to use it, open up the "Getting Started Guide".
Hope this helps!