Extract the "Left 4 Dead 2" folder to your Steam/steamapps/common/ directory, replace all.

To start a game with a certain mutation use the following command:
map <map name> <mutation name> 

Mutation Names:
	chainsaws			(Chainsaws Only)
	fourswordsmen		(Katanas Only)
	hardcore			(If everyone dies, the campaign ends)
	headshotsonly		(Headshots Only)
	lastmanonearth		(1 Survivor vs 4 SI)
	linearscavenge		(Get 2 cans in, 2 more spawn)
	m60s				(M60s Only)
	maxspecials			(Survival with 8 SI at any time)
	nohealthversus		(No Health Versus)
	realismversus		(Realism Versus)
	temphealth			(Bleeding Out)
	ultrarealism		(Ultra Realism - BROKEN)
	versussurvival		(Survival Versus)
	viptarget			(Guard the Gnome)
	mutation3			(Bleeding Out)
	mutation9			(Guard the Gnome Versus)
	madness				(No SI, lots of commons)
	realismscavenge		(Realism Scavenge)
	coopversus			(Play coop as the SI)
	
Descriptions:

	Chainsaws (coop) - Everyone starts with a Chainsaw (unlimited fuel) and a pistol (unlimited ammo), there aren't any other kinds of weapons spawning, so it's just what you have at the beginning.
	Four Swordsmen (coop) - Kinda like Chainsaws, except you start with katanas and no pistols, and there are only S.I., no Commons.
	Hardcore (coop) -If everyone dies, you have to restart the campaign, so no restarting maps, you have to restart the whole thing!
	Headshot Only (coop) - Self descriptive, commons only die with headshots.
	Last Man On Earth (singleplayer) - It's you... against the S.I. (extremely scary)
	M60s (coop) - Survivors start with only M60s (limited ammo) and a Desert Eagle, once you run out of ammo, you're screwed.
	Max Specials (survival) - It's survival, with 8 S.I. at any given time!
	Bleed Out/Temp Health (coop) - Everyone starts with 100 decaying health (pills-like), there are no medkits, only pills and adrenaline.
	Protect the VIP aka Gnome (coop, versus) -There is a gnome next to you at the beginning, carry it through the campaign, person carrying the gnome can't use weapons and the gnome attracts infected!
	Realism Versus (versus) - You already know what this is, versus with realism modifications.
	Linear Scavenge (scavenge) - 2 Cans spawn, get those 2 in and 2 more spawn, proceed...
	No Health Versus (versus) - No medkits, no pills, no adrenaline.
	Versus Survival (survival) - Self Descriptive, it's a lot of fun.
	Madness (coop) - There are TONS of common infected, they are VERY angry and there are no S.I., you have enough trouble as it is.
	Versus Coop (versus) - It's coop, with a twist, you play as the infected as the bots try to make it through the campaign alive.
	Realism Scavenge (scavenge) - Self descriptive, scavenge with all the realism twists. 

All credit goes to:

[EliteGuy/Green]
	Made the gamemodes.txt, tested everything and got it working.
	Author of custom mutations: Coop Versus.
	
[Technopath/DarkDragon]
	Figuring out how to get the gamemodes.txt working, helped making gamemodes.txt and testing.
	Author of custom mutations: Madness & Realism Scavenge.
	
[ThaiGrocer]
	Getting vpk edit working.