Ready to trip knox and install a better ROM, how do I install TWRP?

I want to use this awesome ROM: http://forum.xda-developers.com/gala...n-1-0-t3094423

I need to install TWRP. When I first got the phone, I learned that this was not possible with AT&T. This still the case? I have been searching and I haven't found any tutorials or youtube videos specifically for AT&T Galaxy s6 -- is it the same process? I'm currently running Android 5.0.1 and I flashed a ROM with Flashfire (Ultra Light 1.8 by Rayman360) -- it's a great ROM but you can't do much since it's basically a de-bloat stock.

Can someone point me in the right direction for installing TWRP on the AT&T Galaxy s6?
I suspect I will need to do a full wipe here and then re-root with a different method.

I am familiar with Odin and TWRP, I have been using custom ROMs for years now.

Just ordered one and getting ready

Hi Guys. I took advantage of an ultra good deal and my Nexus player will be here friday. You've read threads like this many times before but I'm not being lazy (just strapped for time with work and my toddler.) So, I'd love to devote more time to playing with my new toy rather than googling everything there is. I have a handle on basics but if there's anything that the current guides don't cover (tips, tricks etc) please let me know.
Also, I love the idea of running CM 12.1. It was supposed to happen on fire tv but never did. However, I'm not crazy about using my bt keyboard for the set top box. I want to use the stock remote. BTW-does mouse toggle app work on this? Is it possible to use leanback launcher on CM 12.1 so we can have remote functionality and the cool CM features?
Really looking forward to this. Anything you think would be useful to get me going like if there is a thread or web page like "Getting started with Nexus player" please let me know. Thanks!