@radiocycle - it's very easy.
As I said above, I used Wug's toolkit to do it.
I just downloaded the latest TWRP and SuperSU.
Then started the toolkit and pointed it at the downloaded TWRP, then hit the ROOT button.
It copies and installs TWRP.
Once done, it boots into TWRP, where I flashed SuperSU.
I have had issues installing latest SuperSU from GP.
I've had to reflash via TWRP each time.