How can I Flash Custom ROM Galaxy SM-G900i without triggering Knox?

A

AC Question

Flash Custom ROM Galaxy SM-G900i without triggering Knox

As I'm completely knew to all this, my questions probably sound pretty dum, I'm sorry, I just want to make sure i do the right thing.
I have managed to ROOT my galaxy S5 which had lolipop and to do that without triggering knox, I had to downgrade to Kitkat. Success.
SuperSU has theoretically disabled Knox, but I'm wondering if that means I can install any Custom ROMs with TWRP for example, or if I need to flash them via Odin?
In truth I'm unable to do either and I would need instructions, but i want to definitely not trigger knox.
I read in many forums that using TWRP triggers it, but is that also if it has been disabled by SuperSU?

I've tried putting the zip file of:
omega-roms/G900I_Omega_Kernel_LL_5.0
inside my internal drive and flashing it with Odin Pro but it says the partition cannot be found in file.

It was so easy to flash Kitkat with Odin Pc and downgrade, can't i do the same with a custom Rom? The files of custom ROMS don't seem to be .tar and they have lots of files inside that i don't understand.
What do I do about the modem?

Also, I have seen a tutorial of how to do it with TWRP but when i start my phone in recovery mode the TWRP interface doesn't come up. It start the usual interface there was before i installed TWRP.
How do I make it take over?

I know there are too many questions, If i could have a clear guide from step 1 of how to do it knowing it doesn't trigger Knox, I'll be ever grateful!
Valentina