How do you downgrade from 5.1.1?

I have an s5 that came with 5.1.1 completely stock, unrooted with baseband: G900TUVU1FOF6. I have never downgraded an s5 before. I've only done it to nexus devices. Does anyone have any instructions on how I should downgrade it? I was thinking of using the 5.1.1 rooting procedure that involves flashing a kernel with kies then maybe it would be possible to flash a custom 5.0 rom with recovery. I'm not sure that is a good idea though because I wasn't sure if the bootloader, CSC, or PDA firmware would still be stuck at the 5.1.1 version.

Edit:
I found this video which seems to say you don't need to root. I'll try the video's method. https://www.youtube.com/watch?v=Nek0lowlMmM