At the Steam control panel, Right Click the "Left 4 Dead 2" option, and select "Properties" under the "My Games" tab. In the General tab, select "Set Launch Options", enter "-console" in that field, and accept it. Start the game, and the console window should appear at the main menu. Enter "bind p toggleconsole" as a console command. Then, type one of the following "map [map name]" codes and press [Enter] to load the corresponding map. Note: The same entries can be used with the "changelevel [map name]" or "changelevel [map name]" codes during the game.

Alternately, at the main menu, select "Options", then choose the "Keyboard/Mouse" selection. Enable the "Allow Developers Console" option. Press ~ to display the console window. Enter "bind p toggleconsole" as a console command. Then, type one of the following "map [map name]" codes and press [Enter] to load the corresponding map:

Dead Center
map c1m1_hotel
map c1m2_streets
map c1m3_mall
map c1m4_atrium

Dark Carnival
map c2m1_highway
map c2m2_fairgrounds
map c2m3_coaster
map c2m4_barns
map c2m5_concert

Swamp Fever
map c3m1_plankcountry
map c3m2_swamp
map c3m3_shantytown
map c3m4_plantation

Hard Rain
map c4m1_milltown_a
map c4m2_sugarmill_a
map c4m3_sugarmill_b (Mill Escape level)
map c4m4_milltown_b (Return To Town level)
map c4m5_milltown_escape

The Parish
map c5m1_waterfront_sndscape (Demo level; will glitch graphics until you quit the game)
map c5m1_waterfront
map c5m2_park
map c5m3_cemetery
map c5m4_quarter
map c5m5_bridge

While playing the game, press P or ~ to display the console window. Enter "sv_cheats 1" to enable cheat mode. If you entered the code correctly, the message "Server cvar 'sv_cheats' changed to 1" will appear. You can now enter one of the following codes to activate the corresponding cheat function. Note: Enabling cheat codes will disable achievements from being earned.
List of console commands
Result /// Cheat Code
Toggle God mode for entire team /// god [0 or 1]
Toggle no clipping mode /// noclip
Toggle Incendiary Ammo upgrade /// upgrade_add incendiary_ammo
Toggle Explosive Ammo upgrade /// upgrade_add explosive_ammo
Toggle Laser Sight upgrade /// upgrade_add laser_sight
Toggle infinite ammunition /// sv_infinite_ammo [0 or 1]
Refill health /// give health
Refill ammunition /// give ammo
Get ammo for primary weapon /// impulse 101
Spawn double pistol ///give pistol
Spawn Magnum /// give pistol_magnum
Spawn auto shotgun /// give autoshotgun
Spawn chrome shotgun /// give shotgun_chrome
Spawn pump shotgun /// give pumpshotgun
Spawn SPAS shotgun /// give shotgun_spas
Spawn SMG /// give smg
Spawn silenced SMG /// give smg_silenced
Spawn AK-47 /// give rifle_ak47
Spawn M16 /// give rifle
Spawn combat rifle /// give rifle_desert
Spawn hunting rifle /// give hunting_rifle
Spawn sniper rifle /// give sniper_military
Spawn grenade launcher /// give weapon_grenade_launcher
Spawn Hunter claws /// give melee
Spawn Boomer Bile /// give vomitjar
Spawn chainsaw /// give chainsaw
Spawn frying pan /// give frying_pan
Spawn guitar /// give electric_guitar
Spawn katana /// give katana
Spawn machete /// give machete
Spawn nightstick /// give tonfa
Spawn Molotov cocktail /// give molotov
Spawn pipe bomb /// give pipe_bomb
Spawn active pipe bomb under you /// boom
Spawn propane tank /// give propanetank
Spawn gas can /// give gascan
Spawn oxygen tank give /// oxygentank
Spawn first aid kit /// give first_aid_kit
Spawn defibrillator /// give defibrilator
Spawn adrenaline /// give adrenaline
Spawn pain pills /// give pain_pills
Spawn a zombie /// z_spawn zombie
Spawn Spitter /// z_spawn spitter
Spawn Jockey /// z_spawn jockey
Spawn Charger /// z_spawn charger
Spawn common zombie /// z_spawn zombie
Spawn zombie horde /// z_spawn mob
Spawn Boomer /// z_spawn boomer
Spawn Hunter /// z_spawn hunter
Spawn Witch /// z_spawn witch
Spawn Smoker /// z_spawn smoker
Spawn Tank /// z_spawn tank
Spawn special Infected /// z_spawn [Infected name]
Level select /// map [map name]
Level select /// changelevel [map name]
Toggle received damage, but cannot die /// buddha [0 or 1]
Toggle if bots do friendly fire damage /// sb_friendlyfire [0 or 1]
Set amount of damage done to survivors /// z_pounce_damage [number]
Set no clipping mode speed; default is "5" /// sv_noclipspeed [number]
Bot teammates shoot nonstop /// sb_openfire or open_fire
Change zombie health /// z_health [number]
Change zombie speed /// z_speed [number]
Set maximum range of Smoker's tongue attack; default is "750" /// tongue_range [number]
Set amount of time before a burning Witch dies from fire /// z_witch_burn_time [number]
Toggle witch to change targets or focus on the survivor that first alerted her /// z_witch_allow_change_victim [0 or 1]
Toggle controllable Boss Infected on any map /// director_no_human_zombies [0 or 1]
Toggle never ending panics /// director_panic_forever [0 or 1]
Force panic event /// director_force_panic
No mob rushes /// director_no_mobs [0 or 1]
Remove all bots /// Ent_Remove
Set damage for a Witch's attack does /// z_witch_damage [number]
Set maximum number of regular zombies /// z_common_limit [number]
Set maximum health of tank /// z_tank_health [number]
Set number of seconds until player loses control of the tank from not attacking survivors /// z_frustration_lifetime [number]
Set probability of infected CEDA agents carrying bile jars /// sv_infected_ceda_vomitjar_probability 0.1
Disable all wanderers, mobs, specials, and Bosses /// director_stop
Third person view /// thirdperson
Over the shoulder third person view /// thirdpersonshoulder
Toggle crosshairs /// crosshair [0 or 1]
Toggle set fixed camera in third person or third person shoulder mode /// thirdperson_mayamode
Disable third person view /// firstperson
Disable all cheats /// sv_cheats 0
Commit suicide /// kill or explode
Quit game /// quit
Video Tutorial
Adding/installing custom maps
First of all, locate your Steam installation directory (by default it is under Program files or Program Files(x86) > Steam > Steamapps > common > Left 4 Dead 2 > left4dead2 > Addons) Once you've found your Steam installation directory, add the map file (.vpk, .zip, or .rar) to the appropriate folder. Next, launch Left 4 Dead 2 from Steam. From the main menu, click on Add-Ons and then enable whichever map you'd like to play by clicking the checkbox next to the name. Click on the appropriate game type for the map and start a server. From there, you should see the map select menu. Click on it and choose Addons, and you should see the map you added if you are in the game type the map was created for. Alternatively you can open your custom map by entering "map [mapname]" in the console.