How to Unlock a locked bootloader Devices

Req.
Your Nexus phone

->Phone model, hardware code name
[Galaxy Nexus, Maguro ; Nexus 4 ; Mako; Nexus 5; Hammerhead; Nexus 7 2012, Tilapia{GSM}, Grouper {wifi}; Nexus 7; deb{LTE,4G}, flo {wifi} ; Nexus 10, Manta; Nexus 6, Shanmu; Nexus 9, Volantis, Nexus 5x; Nexus 6P ; OnePlus X {Onyx} ; OnePlus 2]
PS:One Plus One, A0001 ; Bacon for both [ColorOS/CyanogenMOD] version

-> USB cable
-> fastboot files 
[for Fastboot files, just extract the zip and put it at everywhere]

->PC

and here what did you wanna to do

1st, go to the settings > about phone > tap 7x times of build number till it gets development options
2nd, go back main settings > Development options and enable the USB Debugging
3rd, shut off your phone and hold volume down and power buttons
and you will see this mode, 

This is called Bootloader modes, EG. Nexus 5 

after that, just hold SHIFT + right click mouse and select new command code

Afterwards, plug your phone into PC [let it installing the ADB drivers, if installed failed, Google it, Android Naked Driver and follow up the step for installing naked driver ]

Hence, go for the codes 

>fastboot oem-unlock
>there was popped long and lame lame words at your phone screen..
>just hit pwr btn [choose as yes]
>retype fastboot reboot and check :)
>done , now you're free on locked bootloaders and u may flash any roms/recoveries/ kernels/ tweaks as you wish 
{add-on: after that reflash the system again with the code [cause, once you done your Unlocking Bootloader, it'll wipe whole entire system and data of your phone]
fastboot flash bootloader *name*.img-> flash radio *name*.img -> update -w image-<hardware code name-android version>.zip "Guides for those who unlucky that unlocked his/her phone and switch on the the phone and its bootlooping"}

+YOU had been warned, and once u unlocked bootloader and your data lost, please perform a full backups, if not, I'll not responsible for your loss data or your stuff and i'll LOL'ed at you 

Another method
use Toolkits to unlocking it [Bootloaders]
like MSKIP's toolkits or Nexus toolkits or etc but i recommend you guys prevent USELESS SHIT OF Chinese APPS like MGYUNROOT, 360ROOT (IT MAY FAILURE/RUINED YOUR UNLOCKING PROCESS), these toolkits can be founded on XDA-Developers