Raz it really is quite fairly straightforward (once you have the base setup), just get the latest cap file for the Nuc 8 BE (it looks like it might be this one below)
and update with F7 on boot (from a memory stick)
Then follow the instructions for enabling Legacy mode boot.
Then burn the ROCK image to a memory stick using balenEtcher and boot off the memory stick and follow the instructions.
Just remember to take the memory stick out before you reboot as it will do it all again otherwise.