Drag and drop the file in
...\SteamLibrary\SteamApps\common\DarkestDungeon\mods\Sisters Mod Base\heroes\sisters

Then Rename it "sister_A", "sister_B", "sister_C" or "sister_D" and it will take the place of the skin you overwrite in game.

Don't forget to do a save of the original skin you wnat to overwrite in case you want to use it some times.