Maybe a picture would help.
Barb is the expert here, but I have a picture of what I used as a stabilizer (which in my understanding is needed to keep the pleats from stretching). I just used a piece of plain (pink sorry to say) cotton that my wife had in her scrap pile. The material had zero give to it and it was handy so I used it. It worked fine.



The horsehair stuff I think is used as a stiffener for the apron. I saw the price of that and I chose not to use it. I had some extra (cotton) canvas and I used that. It seemed to do the job well enough.