I bought a book back in 1967 that was the companion text to the classic Italian Documentary “Africa Adio” (the book had the same title) that described the ceremonies. I was only 13 at the time so I was shocked and intrigued of course at the chapter on Kenya.
I suppose if you simply google “Mau Mau ceremonies” you will be able to access the gist of them—though be ready to be disgusted.