This assumes some previous knowledge of using ADB and TWRP.
Please note that although this worked on my phone, I accept no responsibility if you brick your phone doing this
1. Update your device Firmware to the latest version because after the rooting process you cant update anymore as you will have a TWRP recovery. Then, open Settings -> About and tap on Build Number seven times to enable Developer options. Press back, click Developer then check USB debugging & OEM unlocking of the bootloader.
2. Connect your phone to your PC, start ADB and type adb devices – this will bring up a prompt on your phone that you need to respond to. Type adb reboot-bootloader
3. Once in bootloader mode, type fastboot oem unlock. Hit the Volume Up key to confirm.
4. Disconnect the phone from the PC and remove the battery for at least 3 seconds.
5. Download the SuperSU flashable zip here and copy it to a microSD card and insert it in the phone.
7. Start the phone in bootloader mode by holding the Power and Volume Up buttons together until you see white text on the screen. Use the Volume Up button to navigate to Fastboot then select it by pressing Volume Down. Connect the phone to your PC.
8. Type fastboot flash recovery twrp.img
9. Disconnect the phone from the PC and remove the battery for at least 3 seconds.
10. Start the phone in Recovery mode by holding the Power and Volume Up buttons together until you see white text on the screen. Use the Volume Up button to navigate to Recovery then select it by pressing Volume Down.
11. You should then see TWRP starting. Tick the box to agree then swipe the blue button to the right.
12. Tap on Wipe then Format Data. Do not miss out this step or TWRP won’t be able to mount the /data partition.
13. Tap the Back button then Factory Reset and swipe the blue button to start.
14. Tap the back button then Install then Select Storage -> microsd card then your SuperSU file then swipe the blue button to the right to flash it.
15. Tap on Reboot System and your phone should boot to your newly rooted OS! Don’t panic if the first boot takes 5 or 10 minutes. If it gets stuck, pull the battery & try again.
16. Make sure you disable System Updates as they will no longer work with the non-stock recovery.