working root method for 5.1.1 with unlockable bootloader?

I have mine updated to 5.1.1 and the bootloader is unlockable. However, the rooting instruction is confusing for this version.
I am looking for the definite solution for rooting (if it is required to unlock the bootloader, should I backup TA partition first?) 100% stock 5.1.1.
Frankly, I don't want to mess with Kingroot or similar tools from China.