You first of all need to download the mod:
http://www.moddb.com/mods/pezbot/downloads/pezbot-005p-for-world-at-war

Now you need to open the start menu and type %LOCALAPPDATA% and then press enter.

(FOR WINDOWS 8 USERS):
Press the Windows key and then go to search and type in "run", click the first program that shows up and then type in %LOCALAPPDATA%.

You will find a folder called "Activision", open it and then open the folder "CoDWaW" and then the folder "mods". (if you dont have one, create one!)

Now open the downloaded file (PeZBOTWAW_005p), you will see a folder "mp_PeZBOTWAW", copy and paste it (or drag it) into your "mods" folder.

After all that done open your Steam Library and right click on Call of Duty: World at War, click properties and then click "SET LAUNCH OPTIONS" in GENERAL tab.

Select the commands below and copy & paste in it.

Recommended:

+set fs_game "mods/mp_PeZBOTWAW" +set svr_pezbots_weapons authentic +set svr_pezbots 9 +set svr_pezbots_useperks 0 +set svr_pezbots_skill 0.7

What these commands do:

+set fs_game "mods/mp_PeZBOTWAW" <- (Use the mod when opening up the game automatically.) +set svr_pezbots_weapons authentic <- (Makes the bots use weapons based on their faction.) +set svr_pezbots # <- (Number of bots you want to play with.) +set svr_pezbots_useperks # <- (Obviously, use or not ingame perks, 0 = not to, 1 = to use.) +set svr_pezbots_skill #.# <- (Obviously the skill of the bots.)

Now you need to launch the game and to turn off the Punkbuster known as simply "pb" in the
"OPTIONS & CONTROLS" > "MULTIPLAYER OPTIONS". After that done press ESC and click "START NEW SERVER" and turn off again the Punkbuster.

You can customize the game mode, map etc in the "Game Mode Settings".

Now click "Start" right-down to start the game.

(NOTE!)

Every time you quit the match, to play again with bots you will need to open the console with the key: ~ (under the esc) (in case you can't open the console, you need to go in the "OPTIONS & CONTROLS" again and then this time you should click the "GAME OPTIONS" and enable the console.)

Now in console you must type: /svr_pezbots 9 (and press enter).

You're now done!