To make the skin, there are basically two tools you gonna need, those are:

• Insurgency Sandstorm's Mod Tools & Editor, and
  
• An image editor to edit Targa images (.TGA)

Insurgency Sandstorm SDK is available exclusively on Epic Games Store here[www.epicgames.com].
Don't hit me up with "Whoa you will need to make EGS account and download 100GB-ish editor for this!?" Yes.

And on how you will skin the guns with the image editor it's really up to you really, whether it's just a simple 2D skinning with GIMP for example or if you want to edit it in 3D way you could use Blender as well.
Both GIMP and Blender are free, you can get GIMP here[www.gimp.org] and Blender here on Steam even.

So, let's get started from the basic to create a mutator mod so that you'll be able to use the skins in the game.
After you done downloading the editor and launch it, this is how it'll look like more or less:
 
Then, create a new mod by clicking "Create Mod" from the top bar:


Or you can use and download my mod template here, via Google Drive[drive.google.com].

Set the mod folder (without spaces) and set the descriptor to your liking:


After your new mod folder has been created, create a folder to hold a mutator file and other one preferably to store the mod files, personally I like to go with something like this:


Open the ModData, then set the mutator's in-game name and set the asset management to recognize any mutator you'll be making later make sure the directory is the same as your mutator folder inside the mod folder:


If you can't save the changes you've made on ModData, right click on the ModData > Asset Actions > Reload, to reload it.

You can see an example on how the game's DLC weapon skin upgrades work on the editor as a solid reference here in Content\Game\Actors\Weapons\Upgrades\Skins:
Please just use this since as a reference since I'm pretty sure this guide will miss one or two things. If you want to go deeper to know how the game calls in these files, right click the file and choose Reference Viewer (or Alt+Shift+R) to learn their structural tree.
 
To make our own, firstly, create a weapon skin upgrade blueprint:
Right click on the content browser > Create basic asset > Blueprint class
When picking the parent class, personally I like to use the WeaponSkinComponent as the parent
 
And there you have it, a new blueprint for weapon skin.
 
And for example I'll skin the AUG since personally I think it's the easiest gun to skin.
The blueprint will be referenced by others on the editor, I like to name it in the same format as what the dev uses as well, so here goes BP_UG_Skin_AUG_Example.
 
Things that you'll need to work on that blueprints are basically just setting up the weapon material overwrite parameters and it's weapon upgrade description parameters.
 
On the material overwrite parameters, you need to assign which texture slots on which materials goes into. Both for the first person and the third person model. The weapon's skeletal mesh should show you that, they are located on Content\Animated\Weapons.
This is AUG's (Content\Animated\Weapons\Rifle\AUG) for example:
 
And as you can see, in both first person and third person model AUG only has one material element in slot 0. So then later you can adjust the material overwrite parameters for the AUG in the earlier BP_UG_Skin_AUG_Example this way:
 
Weapon index material overwrite 1P as for the first person model, assigning it's slot to 0 carrying the 1P weapon skin material.
Weapon index material overwrite 3P as for the third person model, assigning it's slot to 0 carrying the 3P weapon skin material
And also reference it's original material as a fallback mesh material as well, that are located in the same folder as the weapon's skeletal mesh

Material overwrite will just replace the gun's "main body's" texture to your custom texture, the fallback material replacements can also replace the textures of some model that uses the same material instance of that gun, usually upgrade attachments. So check the boxes for "Check Upgrades for Fallback Material Replacments", you can also individually assign which upgrades will be affected, if not also check "Check All Upgrade for Fallback Skins". For this example that will affect the AUG's modular scope, you can also check what material instance what upgrade uses by opening up their model as well, they are located in Content\Game\Actors\Weapons\Upgrades.
 
And for the replacement materials, I forgot to add that you need to make them first, simply by copy and pasting both 1P and 3P original materials into your mod folder.
 
Editing them will be covered in the next part. Hang tight, the order list of this guide is messed up.
 
Getting back to the weapon blueprint, for the weapon upgrade parameters you will need to name the skin, it's description, set it's supply point costs, set it's upgrade slot to aesthetic, and set what weapon it's intended for. Also make sure you uncheck "Is Unlocked by Item ID" since that will make the skin non-useable.
 
That should be all for the skin blueprint, at least that's what I've done to my mod and it worked surprisingly alright.

Back on the copy pasted weapon material, you can replace three inputs of albedo map, normal map, and it's material map.
If you want to keep the mod size simple it's usually enough to just replace the albedo map while retaining the default normal and material map.
 
To export the default albedo map for editing, go back to the folder where the weapon material located, and go inside texture folder under it (or just click the magnifying glass icon under the default albedo input).
Right click on the albedo map > Asset actions > Export...
It will then export the texture as a targa (.TGA) file.
 
If you want to edit the skin in 2D way with GIMP for example, just open it with GIMP then do your own magic on it. I can't teach graphic design so I can't teach ♥♥♥♥♥♥♥♥♥ for you.
Tips? Uhhhhh well you can just play with hue shifting or convert it to black and white to convert tan guns to black, that's what I've done.
 
If you want to edit the skin in 3D way with Blender for example, you will need to export the gun's 3D model.
Right click on the albedo map > Asset actions > Export...
It will then export the weapon 3D model as a filmbox (.FBX) file.
Again, can't teach you how but there's this cool video on how to do texture painting in Blender:
Link for the video if embed viewer is broken here.
 
After you've done editing the textures(s), reference them into your own material files.
 
Then reference this material in your weapon skin blueprint.

 
If you want to make certain parts of the gun glows in the dark, check the Welrod, it has an emissive map for it's sights, it's a pretty good reference to learn.

After you've successfully created your weapon skin upgrade blueprint you will need to include it as a usable upgrade. Here's how you can do it on yours, in a simple guide though, to make a custom theater mutator mod.

In Insurgency Sandstorm, there's these "layers" of: Theater, Faction, Squad, and Playerclass.
Theater holds the information of what squad a human players or bots will use per faction in a certain game mode.
Faction, well, there's Insurgents and Security as defaults.
Squad holds what player classes a faction can use during the game.
Player classes holds what equipment can a player use while using that class, these are rifleman, breacher, advisor, etc.

Create a mutator blueprint:
Right click on the content browser > Create basic asset > Blueprint class
Then use Mutator as the parent
 
Rename it to whatever you like, keep in mind this will be the mutator's name that'll be called in when setting up a game:


Set it's display name (will be visible on the game menu when pressing Tab), author, and description to your liking:


Copy the vanilla theater from Content\Game\Factions\Theaters into your mod's file directory:



This is what the theater asset look like, in here it's kinda up to you if you want to replace the faction definition or just change the squad layout used for a certain game mode, but as an example so that everyone can use the weapon skin on all mode let's try to put the weapon skin blueprint upgrade as an allowed upgrade inside the faction definition:


For that, again, copy paste the Security and Insurgent's faction definition asset into the mod directory:


Inside the faction definition asset, scroll down into the allowed equipment tab and you can put in the custom skin weapon blueprint as an allowed weapon upgrade:


You might also want to consider making a PlayerEquipmentCollection asset to hold all your custom skins, to put in inside the faction asset as a collection instead of listing them in one by one:



Back track and make sure you reference the custom skin under your PlayerEquipmentCollection, referred by your faction definition asset, referred by your theater definition asset, that's referred by your mutator. Layers by layers like a lasagna eh?



Save all files,and recheck everything.
Vanilla mutators like BoltActionsOnly or CompetitiveLoadouts might also help as a reference on how to work on a loadout mutator as well. They are located in Content\Game\Mutators on the editor.

After you're ready click on "Share Mod" on the top bar of the editor, then sign up or log in into your mod.io account:


Set the mod name, name ID (will be used as your mod link on mod.io, no spaces), summary description and tags if you want, but use a large enough picture as the preview image (1280x720+? else it usually returns a HTTP error code 422), and set the visibility to public so that you can use the mod later in the game.


Click on "Add Mod" and wait for the editor to compile it. After it's done, subscribe to the mod from mod.io, and make sure the game downloads it.
As an example to run it in game on local play, do a local play on any map then write in and execute:
Open Farmhouse?Scenario=Scenario_Farmhouse_Range?Lighting=Day?mutators=XXXXX
On the game's console (`) to run a firing range with your mutator mod loaded in.

Also, I don't know if this still happens on the latest version of the editor but I had to move the mod folder from Epic Games\SandstormEditor\Insurgency\Mods into Epic Games\SandstormEditor\Insurgency\Plugins before compiling it.

Other than GIMP and Blender, my friends who've done weapon skinning for our mod used Photoshop and Substance. There's a lot of other app for this, so get yourself to what suits you the most.



 
Go wild with your imagination, really.

Yutapon's "Shigure" M870
From the Azur Lane series

 
Lukemoondoe's "Sandstorm" M4A1
Also comes with tan accessory upgrades

 
Seraphy's "Tiro Finale" Mosin Nagant
I haven't actually watched Madoka Magica

 
Nao's "AstolFAL" FAL
Mastah u ghey

 
Zima's "Code Red" MP7
Glows in the dark as well

 
"Flora" AUG
It blends in on the flower field, really

 
"Albus" Mk17
Only works in Bab