Copy the mod files to the folder:
[game installation folder]\Hellish Quart Data\StreamingAssets


for example:
C:\Steam\steamapps\common\Hellish Quart\Hellish Quart_Data\StreamingAssets

or

C:\GOG Galaxy\Games\Hellish Quart\Hellish Quart_Data\StreamingAssets

The mod will be available as character custom skin in the Character Select Menu.