There's a newer, up to date text guide here.
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2712310851

If you ever wanted to create weapons for L4D2 you probably searched for a bunch of tutorials.
Maybe you found a few. One for soundmods, one for hud images, one that uses the $includemodel way for animating and others that mention an IK rig but you couldn't get a working IK rig in blender because you just weren't able to do it. Or too lazy.

My tutorial covers:
Creating view and worldmodels
Animating viewmodels
Creating a Hud image
Creating a soundmod

You also get a downloadable file which contains:
A photoshop save for hud images
Hud image prefabs in all possible sizes (large gun, large melee small items and chainsaw)
Hud script part prefab for all sizes and uses
A blender save file with viewmodel arms and set up IK rig
A list of all usable soundevents for animations
A prefab Addoninfo file
A textfile containing premade $animation lines for idle, lookposes and run

Prefab: https://drive.google.com/open?id=1Mvxyu12ZP0dCL1B8ycvqo4J8onRbG5y8
Custom content folder guide: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1507939076
Attachments guide: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1464152705
Right-click to VPK guide: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1544981726
Animation Framecount and FPS guide: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1483047118
$Includemodel tutorial; for when you don't want to make your own animations.
The part between 24:22 and 39:20 can be substitued with this tutorial:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1946053787

Crowbar: https://psteamcommunity.yuanyoumao.com/groups/CrowbarTool
VTFEdit: http://nemesis.thewavelength.net/index.php?c=178
https://gamebanana.com/tools/6801
Original website down. This is an archive of all the developers relevant tools, Containing VTFedit.
Blender 2.77 (newer versions don't work nice with smd plugin): https://www.blender.org/download/releases/2-77/
Blender smd tools: http://steamreview.org/BlenderSourceTools/
Notepad++ https://notepad-plus-plus.org/download/v7.6.3.html
Notepad ++ VDF languages (shows syntax highlighting): https://developer.valvesoftware.com/wiki/Notepad%2B%2B_VDF_languages

For textures you can use photoshop, gimp or paint.net with the vtf plugins.
Alternatively, you can use VTFedit to manually convert textures into VTF Format one by one or in a batch.
Potoshop vtf Plugin (32bit version ONLY) http://nemesis.thewavelength.net/index.php?p=39
Gimp VTF Plugin: https://github.com/Artfunkel/gimp-vtf/releases
paint.net vtf plugin: http://nemesis.thewavelength.net/index.php?p=50

Timestamps (they actually work on youtube):
0:40 - Gathering original files
1:58 - Decompiling models
2:15 - Bender - importing files
---
Worldmodel
2:45 - Scaling
3:50 - Rigging
5:42 - Checking materials
6:09 - Fixing smoothing
6:55 - Collisionmesh
7:30 - Exporting model
8:45 - QC Editing
10:48 - Compiling for HLMV
11:53 - Converting textures to VTF
13:05 - Creating VMT files
14:05 - Pasting textures into game files' mod folder
14:30 - Attachments
16:16 - Compiling for L4D2
16:48 - Making test file
---
Viewmodel
17:43 - Preparing viewmodel
21:19 - Rigging
24:14 - Exporting model
24:22 - Preparing idle animation
24:42 - Animating tips
28:21 - Posing viewmodel idle animation
30:46 - Exporting test idle animation
31:01 - QC File preparing
32:33 - Attachments
33:08 - Idle animation
34:20 - Lookposes
35:24 - QC file animation line preparing
38:10 - Animating other animations
---
Hud image
39:20 - Creating camera for rendering
40:19 - Texture placement and Script editing
41:04 - Creating HUD image from render
---
42:34 - Soundmod
---
45:34 - final testing

Good luck creating your own mods.

Sometimes working on a viewmodel ends up with playermodel arms looking like a balled up mess.
This is caused by having a different $definebone line order. Either by writing new $definebone lines in a different order, or deleting existing $definebone lines.
It can be fixed by using the original $definebone lines and not modifying their order at all.
You are allowed to modify their location and rotation if required.

If you do need new bones, build new $definebone lines by Commenting all existing definebone lines, writing $bonemerge lines for the bones you want and compiling the QC with Crowbars "Definebone option". Then pick out only your custom $definebone lines and append them after all original $definebone lines.

I forgot to mention, but if you are going to make a melee weapon, your VMT files require teh following lines to allow bloddy hands


hunter_01_detail.vtf is the default blood. but you can replace it with a vtf fit for your model if you want.