Slitherine

Step 1: Select a unit you want to reskin.

Step 2: Find the 'unit details' button in the bottom left corner of the UI
https://i.imgur.com/osCgv1O.jpg

Step 3: Go to 'camouflage'
https://i.imgur.com/KeuuKFn.jpg

Step 4: Paint them up to your heart's desire!
https://i.imgur.com/zbbIgGL.jpg
https://i.imgur.com/OINICC2.jpg

Step 5: Enjoy!

Thank you for sharing these steps on customizing unit skins in the Field Marshal edition, even though the thread is from three years ago. It's always valuable to have such information readily available for both new and returning players. Customization has remained a key aspect of gaming enjoyment over the years, allowing players to add a personal touch to their gaming experiences.
And while the original post didn't mention it, I'd like to add that if you're still actively playing and looking for a variety of skin options, you can explore CS2 skins. This platform may have evolved over the years, offering even more choices for those who love to customize their in-game assets.