Hi,
I have prepared a USB key Boot 16 Gb Install Mac OSX El Capitan 10.11.6 on my old Mac Pro 1.1 I bought in 2006. It works great !
If you are interested in getting the new USB key, you can contact me (nanou6835@yahoo.fr) and I can help you by sending it to you along with easy accurate explanations, even for a very beginner.
just have a look at the top of the page at my Mac pro 1.1 (2006) setup with Mac OSX El Capitan 10.11.6