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BEFORE STARTING ANY OF THE STEPS OF THE ADDON CREATION PROCESS, MAKE SURE YOU HAVE DOWNLOADED THE LEFT 4 DEAD 2 AUTHORING TOOLS FROM STEAM.
To do this, follow these 4 steps:

• Click on "Library" on the top left corner of the Steam application window, and then select "Home" from the dropdown menu
  
• Click on the Box above the search bar on the upper left hand side of the Steam window, and click on the row which has the title "Tools" to make sure it has a blue check mark on its left side
  
• Use the search bar above the list of applications or scroll down until you see "Left 4 Dead 2 Authoring Tools"
  
• Right or left-click on "Left 4 Dead 2 Authoring Tools" and then click "Install" and go through the process to install the tools

Downloading the Left 4 Dead 2 Authoring Tools will allow you to package your addon folder for use in-game and upload your addon to the Steam workshop, so they are essential for this tutorial.
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You will first want to download 3 programs before you start creating the addon:

• Image Editor like Photoshop - [https://shorturl.gg/Cf2y] or GIMP - [https://shorturl.gg/4cmy]
  
• VTFEdit - [https://shorturl.gg/lBVFZu]
  
• GCFScape - [https://shorturl.gg/apzr]

You may also need to download these tools if GCFScape or VTFEdit have problems:

• Microsoft Visual C++ 2010 SP1 Redist. Package x86 (32 bit OS) - [https://shorturl.gg/ciBpX]
  
• Microsoft Visual C++ 2010 SP1 Redist. Package x64 (64 bit OS) - [https://shorturl.gg/f63Y]
  
• Microsoft .NET Framework - [https://shorturl.gg/GOHVkf]

You can find the skin files by going to steam → steamapps → common → Left 4 Dead 2 → left4dead2 and opening the "pak01_dir" file in there, and then finally going to the "materials" folder in here. This guide covers editing .vtf (Valve Texture) files, so only focus on that file type.

Now create a folder path leading to the skin file(s) you want to replace. For example, if you wanted to replace the textures for the Machete, you would create this folder path: ([your addon name] → materials → models → weapons → melee) Also make sure to include the specific skin files you are going to replace within your addon folder path, in their proper locations.

Afterwards, find the texture that you want to replace the default texture, extract it from or drag it out of pak01_dir, and have it ready for the next part of the addon-making process.

• Open up VTFEdit and then go to File → Open or drag the base skin file that you want to replace into the program.
  
  
• Go to File → Export and save the skin as a Targa (.tga) file to wherever

• Open your image-editing program and load the Targa of the skin you want to replace into the application
  
  
• Make whatever changes to the skin that you feel like, and then export your file from the program as a Targa file

• Open up VTFEdit and then go to File → Open or drag your custom skin file into the program.
  
  
• Go to File → Save As and save your custom skin as a .vtf file into its proper location within your addon folder, and make sure it's named the same as the base skin it's replacing

• Drag your addon folder into vpk.exe, which is located at steam → steamapps → common → Left 4 Dead 2 → bin → vpk.exe (I recommend making a shortcut of vpk.exe somewhere that's it's easy and convenient to access) A .vpk (Valve Package) file of your addon folder will then be created in the same area that vpk.exe (or its shortcut) is located
  
  
• Drag the .vpk file of your addon into the folder "addons", located at steam → steamapps → common Left 4 Dead 2 → left4dead2 -> addons
  
  
• Test your custom skin in-game

Addon Details

• Create a text file called "addoninfo" and set it up like this (copy&paste text block below):
  
  "AddonInfo"
  {
  addonSteamAppID 550 ------ (don't change; ID is specific to Left 4 Dead 2)
  addontitle "title goes here"
  addonversion version number goes here (1,2, etc.)
  addonauthor "author goes here"
  addonDescription "description goes here"
  }
  
  
• Place the addoninfo file into the main folder of your addon (e.g. addon name → addoninfo)

Addon Image

• Find an image of your choosing (jpg and png for example) and name it "addonimage"
  
  
• Place this image within the main folder for your addon

Open the Left 4 Dead 2 Authoring Tools Steam app

Click the "+" symbol or go to File → Publish New Add-on

Select the .vpk file of the addon that you want to upload to the workshop, title the addon, and add an image for it. Add a description and tags as well if you feel like it

Accept the terms of the Steam License Agreement and click on the "Upload" button on the in the bottom right area of the window