How do I unlock the bootloader on an LG K120 (K4 LTE 2016)? I got a used LG K4 off eBay and I'm looking to flash a Marshmallow ROM. I tried downloading the SuperSu ZIP file and booting into recovery, but the only option was factory reset. 

I rebooted and tried Kingroot, because you don't need to boot into recovery for it to work. It didn't work, it just said 'Root failed'. 

I did some research and found out that you need to unlock the bootloader. I didn't know this because on my previous phone the bootloader was already unlocked. I searched on how to unlock it on my device but all the suggestions just told me to boot into recovery.

Is there a good way to manually unlock the bootloader? I have a raspberry pi 3 and my mother has a Windows 10 laptop, so either can work. Thanks!
LG has agreed to provide bootloader unlocks for a certain few models.  Google  bootloader unlock. Is an  officia LG site. Good luck!