In the file section you can find an archive containing a few custom skin/face examples.
To view/extract existing skins you can use AssetStudio (open Garden Paws\GardenPaws_Data\resources.assets).

Faces only require a .png with the face to be placed in the CustomTextureLoader\Faces folder.
If you want to customise which animals can use the face place a .json file with the same name along side of it.

Example .json file:
[
  {
"animal": 0,
"can": true
  },
  {
"animal": 1,
"can": true
  },
  {
"animal": 2,
"can": true
  },
  {
"animal": 3,
"can": true
  },
  {
"animal": 4,
"can": true
  },
  {
"animal": 5,
"can": true
  },
  {
"animal": 6,
"can": true
  },
  {
"animal": 7,
"can": true
  }
]

  0 = Cat
  1 = Bunny
  2 = Bear
  3 = Fox
  4 = Badger
  5 = Chipmunk
  6 = Dragon
  7 = Bird

Skins have to be placed inside of the Skins folder and require a .png for the texture and a .json with the same name. If you want to have a custom preview image for your skin place a .png with the name of your texture followed by _Preview (e.g. CustomSkin_Preview.png) alongside the other two files.
The only values of .json file the should be changed are: HasHideShow, HideShowName, visuals (pick a number from the list above to decide which animal can use the skin), IsDefault, CanChest, Glittered, BuyPrice.

Example .json file:
{
  "SkinTexture": {
"m_FileID": 0,
"m_PathID": 0
  },
  "SkinPreview": {
"m_FileID": 0,
"m_PathID": 0
  },
  "SkinPreviewImageName": "",
  "index": 0,
  "_ID": "",
  "SkinName": "",
  "HasHideShow": false,
  "HideShowName": "",
  "visuals": 0,
  "IsDefault": true,
  "CanChest": false,
  "Glittered": false,
  "BuyPrice": 0,
  "TextureName": ""
}