Making Skins (before official support)
Pavlov is getting in-game official skin support! Yay! But in the meanwhile we can't upload or play with custom skins in-game in multiplayer. However, that won't stop you (yes, you) from making and testing skins in the meantime. To make skins (And not share them in-game) you don't need UE4, or any of that Workshop Tools stuff from the other guide, but some knowledge of 3D design (specifically textures and materials) would be highly advantageous. Don't worry though, if you're a complete beginner this guide should introduce you to a set of free tools which you can learn to use. More experienced artists will know where and how their software of choice can be used.
Creating a skin
Extracting the original resourcesThe first step in making a skin is to extract the original model and textures. This can be done with UE Viewer by Gildor. You should extract this into a convenient location, then run it.
The next window is the most important, and where most of the magic happens. However, it's a lot more self-explanatory (hopefully). The folder browser is on the left, and files in the selected folder are on the right. Most game files are unsurprisingly found in the Game folder, and the weapon models (which we want) under /Game/Weapons/. Inside this folder will be a folder for each weapon, some still in development. Open the folder of the one you want to make a skin for. I will use the 1911.
When you export an asset using UModel, it also exports any dependencies. This means that we only need to export the model to get the model and the textures. The model file for the 1911 is called "SK_C1911.uasset". The prefix here, SK_ indicates that this file is a skeletal mesh. There are some other files appended with PhysicsAsset and Skeleton. These are the files for the Physics Asset and the Skeleton, not the base model. The SM_ files are unanimated Static Models. You might want them in order to skin bullets and magazines. To extract the desired files, select the ones you want and select Export. In the next popup you shouldn't have to change any settings, just press OK. UModel may now ask you for the version of Unreal Engine used to build the game. At the time of writing this is 4.21 for the current version of Pavlov (and 4.15 for some older Pavlov versions). If there are no crashes you can now close UModel.
Loading the resources in Blender
Step 0 of using Blender is to install Blender. The model file has been exported as a psk file so you also need to import and enable the psk plugin. The video snippet here will help you if you are new to Blender. Once installed, the importer can be found under File->Import->Skeleton Mesh (.psk). Before importing, clear the scene by pressing the A key to select all, then pressing the del key to delete everything. You may be prompted to confirm deletion. Next, import your skeleton mesh file (.psk) from the UmodelExport folder inside wherever you extracted Umodel to. You've done it! Almost... Maybe take a breather if you're new to Blender because the next bit might be a headache.
This weapon model should include material and UV data, but won't know which textures to use.
To select the textures you must go to the material settings. First you must select the actual model. This is a child of the main object that has the suffix ".mo". Then, go to the materials tab. For each material that isnt just named "Material" (most weapons only have one actual material). Find the matching BaseColour texture file, and set it in the textures tab. This tutorial will walk you through that.
When the materials are set up you need to add a light. I suggest a sky (hemisphere lamp). At the bottom of the 3D view go to Add->Lamp->Hemi. Now switch to material view to see the textured model. (The button should be a white circle at the bottom of the 3D view) If at this point the model is not textured you have done something wrong. Now would be a good time to save the blend project. File->Save
Making / Editing the skin
For beginners I recommend simply opening the BaseColour texture file in an image editor that you are comfortable with. If you've only ever used MS Paint, Paint .NET is a good upgrade. Some sort of clever folder structure or version control might be useful here but simply making occasional backups and saving over the BaseColour file will work just fine. When you want to see your changes in blender just refresh the texture. Change to the UV/Image Editor in blender (the button that looks like a white cube in the bottom left), select the texture that you have changed, then press ALT+R. You can then change back to the 3D view.
Sharing a skin
The Really Easy Way
Render a photo out in blender, or if that is beyond your skill level (You should try to learn for fun), take a screenshot with the Microsoft Snipping Tool.
The Easy Way (Static weapons)
Well, for decorative skinned weapons the process is fairly simple. You can use the static weapon model in any map as a normal asset. (For making custom maps see Getting Started) Just export the model from blender then reimport it into UE4 as you would any other asset. I recommend this video tutorial for beginners as the example asset used also has an ORM (Occlusion, Roughness, Metallic) texture.
The Hard Way (Interactive weapons)
For starters, you can test out the template map on the workshop here. The source is available here. Feel free to use it as a template or rip the blueprints out of it. If you're familiar with UE4 you should be able to figure it out, but if not what you have to do is import the textures as you would for the easy way, and create a material out of them. You can then place a BP_SpawnSkinnedWeapon, select the material that you just created, and set the weapon ID according to this table. The map can then be uploaded like any other custom map. (See Getting Started) Note that local staging of custom gamemodes has been broken by the zombie update, so you will actually have to upload the map to the workshop. To do this you will have to rename the UGC1964010679 to something stupid like "Potato", generate a UGC with the map tools, move all of the assets inside "Potato" to the new UGCXXXXXXXXXX folder (Xs are numbers), right click "Potato" and Fix Up Redirectors, delete the "Potato" folder, then change the description file to match the new UGC number.