Google is an useful thing. Its not hard install mod engine then throw this mod into the mod folder from a site called Nexusmods.com (just search Okita in the search tag) All it takes is some reading. I'd say you should be done in 15 minutes. Good Luck Friend
There's something grotesquely and comically disturbing yet somehow completely in character about Okita in Sekiro and I can't quite put my finger on what it is.