BEFORE YOU READ ANY OF THIS TUTORIAL, know that these custom models will NOT work online. Do not ask why as it was Valve's decision to make custom skins impossible to do. :/

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Programs/Files Needed:
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-Notepad++
-GCFScape
-Decompiled Survivor/Infected Models (Optional)

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Getting the Model Ready/Compiled
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1. Assuming that you already know how to rig character models and you are using the basic l4d bone structure; Start by exporting your model as an .smd and set the .QC file up. Make sure the name of the new model is the same as the old model.
- Download the Decompiled Survivor/Infected Models to examine their .QC files;
then edit it to match settings for your new model.

2. Compile the model. Check to see that the model compiled and is in the correct left4dead folder.

3. Edit the .vpk to change the model to the new one. (Read below)

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Editing pak01_dir.vpk
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1. Open up pak01_dir.vpk in Notepad++.

2. Scroll down to line 40621; where the texture information ends.

3. Search the model name you would like to change.

4. Once you find the model name, change it to anything. Search the model name again and again and keep changing the names so you will eventually get the .phy, .mdl, .vtx, and the .vvd.
- This makes the game not be able to find that model, so it goes to search your game
directory for the model.
- MAKE SURE you do not change v_boomer or something like that.

5. Open up the pak01_dir.vpk in GCFScape and browse to the model you wanted to replace and make sure the name has been changed.

6. Start up L4D and enter sv_consistency 0 in the console before playing. Then test out your new skin! :D
- IF you want to play your with your new skins online, you can only play with them on servers that have sv_consistency set to 0.