To begin skinning, go to Settings -> General -> Folders -> Open fluXis folder, or navigate to:
C:\Users\{username}\AppData\Roaming\fluXis\ ~/.local/share/fluXis/and create a folder named skins.
Inside the skins folder, make another folder for your skin, and make a file named skin.json in that folder. More about skin.json can be found here.
Skins can also provide an icon for the skin selection menu. This icon should be a PNG file named icon.png and placed in the root folder of the skin.
Your skins folder should look like this:
skins
└── [Skin Name]
├── skin.json
└── icon.png [optional]
[!IMPORTANT] Any images provided in the skin should NEVER exceed 4096x4096 pixels in size due to performance reasons.
There are 8 different sections that go into each skin, with their respective folders.
To modify elements in that section, create a folder in the root of your skin with the name of that section.
For example, if you're skinning HitObjects and Lighting:
skins
└── [Skin Name]
├── [...]
├── HitObjects
│ └── [...]
└── Lighting
└── [...]