Hello everyone.
With the new skin workshop added recently I figured I'd have some fun with it. But unfortunately as you've probably realized you can't (by default) test out your weapon skin in-game. I'd previously been doing a bunch of experimenting and hacking with CSGO for a seperate modding project I'd been working on and have become pretty familiar with a good bit of the inner-workings of the game.

If you want some proof or would just like to see how it looks, here are some screens:

*UPDATE 3*
For those complaining that this tutorial is "too complicated." It is not really, 2 steps at most that are quite simple if you can follow straight-forward instructions and are literate in the English language (if anybody would like to repost translations of this, feel free.) If you've made a skin and want to try it in-game... and that is your attitude: give-up skinning now. You're obviously too lazy to make anything anyone would want to ever use, besides perhaps a few extremely loyal friends. Just a few months ago this process was at least 10x as tedious, and you would have never had this opportunity.... feel lucky it's actually this simple now.

So anyhow, I'll try and keep this as straight-forward as possible. Also note, I am not reponsible for anything that goes wrong with this process so proceed at your own risk. Make sure you back-up anything you change. As a last resort if anything is still messed up you can use the 'Verify Integrity of Game Cache' function in Steam for CSGO to restore everything back to default (including any files you may *not* want replaced.)

Also note this will probably only work in a local (bot) game and most likely you won't be able to connect to any games with your files modified.

*UPDATE 2*
Matchmaking won't allow for modified script files (you'll be kicked and forced to abandon match until you change the script file back,) however about 50% of the community servers I tried on (VAC secured or not) seemed to work (at least for the 10 minutes I played on them.)
Previously it seemed to work on all community servers I tried (never tried MM,) so it's possible VAC may have been recently updated (no clue.)

Anyhow...

What you will need:
-text file generated from the workbench for your weapon & the pattern VTF file if your weapon uses one.
-text editor (any)
-a default market skin for the weapon your skin is for (ie: one of the official valve skins)



Step 1 (skip this if your skin doesn't use a pattern texture):
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Goto your CSGO root folder. Navigate to your 'materials' folder. If the following folders do not exist (by default they do not) you will have to create a series of folders inside one another.
Create a folder named 'models' inside your materials folder.
Inside of the new 'models' folder, create another named 'weapons'
Now inside of the weapons folder, create yet another named 'customization'
Now inside the customization folder, create another named 'paints'

Now, depending one which finish style you've chosen for your weapon you will create one of the following folders, depending on which style you're using:

Anodized Airbrush -> 'anodized_air'
Anodized Multicolor -> 'anodized_multi'
??? -> 'antiqued' Note: I haven't tested, but I'm guessing this is for 'patina' style.
Custom -> 'custom'
Hydrographic -> 'hydrographic'
Spray Paint -> 'spray'

Finally, place your pattern texture VTF inside the folder appropriate for it's style.

NOTE: If you'd like to use one of Valve's pattern VTFs they can all be found inside the PAK file following the same folder paths we just created. Extract them with GCFScape.


Step 2
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Now I'm assuming you already have a default Valve skin for the weapon you're skinning. We will be replacing it with our own.
For this example I'll be replacing the hideous SG553 'Ultraviolet' finish with a quickly-made deserty digicamo design.

Navigate to your root "csgo" folder. From there go into your 'scripts' folder. Now find the folder named 'items' and open it. Inside you'll see a text file named "items_game.txt," open it up.

At this point you'll have to search for the skin that you're going to replace, but they aren't necessarily named what you would think. Knowing which collection the skin comes from can help to find the text you're going to need to replace. For this I'm using the UltraViolet skins from the default CS:GO collection.

Near the bottom of the text file search for a string "item_sets" (with the quotation marks.) Within the brackets the skin you're wanting to replace should be listed in the set it comes from (nuke/office/dust/bravo/etc.)
For the skin I'm replacing in this example, I've found the set ""set_weapons_i" and found the SG553 skin from that set, it is named: "so_purple" in the text file.

NOW. Search for that name we just found in the same text file. It will probably be found many times using the search function, but there's only 1 specific occurance we want to look for, it will be preceeded with the text "name", and should be inside brackets with a number index. For the weapon I'm replacing the 'paint kit' number is "98." (Note: Some weapons use the same pattern/texture, so you may need to do a bit of extra checking. If the first one doesn't work try the next one, etc.)

Open up the text file that was generated by the workbench/skin editor for your skin and copy everything *in between* the two curly brackets '{ }'. Now go back to the 'items_game' text file to where we found the paint kit we want to replace with our own skin.
Delete everything inside of the curly brackets for the paint kit you're replacing, EXCEPT for the "name", "description_string" and "description_tag" entries. Now paste the text from your custom skin inside of the curly brackets.
Find the line named "pattern" (if your skin uses a pattern file) and remove all directory and drive/path information and the file extension (.vtf) leaving only the filename in the "pattern" key value. (IE: If your "pattern" key is "C:\patternfile.vtf" you should change it to simply "patternfile")

Now save the items_game file and close it up.
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At this point your custom skin should now be working in-game. You will *NOT* see your skin in the inventory or load-out menu as it will still display the skin as it should be. Make sure you've equipped the skin you replaced.

Now launch a game, and your skin should show up on the buymenu and in both 3rd and 1st person as you'd expect...
FIN!

Sorry for the lack of detail in these instruction. If you need any clarification on any part be sure to leave a message and I'll at least try to help.

Also note: you can extract and hack-in any of the market skins from pak file and use them for free! But I probably shouldn't mention that part :P You can figure that one out on your own.

Cheers.