To create custom, non-replacement skins compatible with PSM, you will need a texture that was cooked via Unreal Engine 4.24. This guide will outline how to do just that.
A link to Unreal Engine, alongside other useful tools, can be found on the Useful Tools page. A light build for Unreal Engine 4.24 is also accessible here.
Unreal Engine 4.24 is required for this process to work! Project Wingman does not support any other version of Unreal Engine, such as Ace Combat's 4.18.
The easiest image format to use for skinmaking is TGA (Targa). Ensure you have exported your image as a .TGA file before you proceed.
To cook your textures, you'll need to create a project in Unreal Engine. When you start the engine, you should be presented with a startup screen with "New Project Categories" options at the bottom. Select "Games" and click Next. Use a "Blank" template, and click Next again. Finally, leave all options on their default settings, except starter content - click With Starter Content and switch it to No Starter Content.
Give your project a name, note the folder it will be located in (you'll need to navigate to that directory later), and click Create Project. With that done, you are essentially ready to cook your textures.
When your project opens, you'll be presented with a large editor interface. You can ignore all of this except the Content Browser at the bottom. Click Import and select your texture. The default settings should be fine; name the texture however you need and simply click Save All after this. Then click File at the top left and Cook Content for Windows. Alternatively, you can drag-and-drop your texture into the Content Browser to import it.
When the cooking process is complete (indicated by two "beeping" or "pinging" sounds), navigate to the aforementioned project folder. Within the folder, navigate to Saved\Cooked\WindowsNoEditor\[project name]\Content and locate your cooked textures. You should find one .UASSET file, one .UBULK file, and one .UEXP file for each .TGA file you cook. Those three .U* files are what you need to pack a PSM-compatible skin (or any skin in general, for that matter).