The XDA devs original instructions are based on SIM unlocking via nv_data.bin and will only work on a rooted device. You’ll need to extract nv_data.bin and use a hex editor to take the hashes from a number of offsets and find the password.
From then on it should be plain sailing as all you need to do it insert your unaccepted SIM and then enter the unlock code when prompted.
For much more detailed information including troubleshooting, head on over the forum thread.
NB Odia gives credits to pulser_g2 and the Fall Guy for their help.