Bunch of tools used for the Lumia 535
Download the files from the releases tab, choose which EFIESP you want: Devmenu only is just to unlock, the EDK2 one is for booting Linux and such.
Open Windows Phone Internals, either from the downloaded .zip, or from the official WPinternals GitHub. Go to the Flash section under Platform.
Scroll down to "Flash separate partitions", and select the EFIESP file to flash.
After flashing, WPinternals will reboot the phone. Press and hold Vol- to get into Developer Menu (before the Windows Logo shows up). be
When in Developer Menu you will see a few options, the one that interests us is the USB Mass Storage option, so press the power button, and you're there.
Close any WPinternals window, and open the Unlock.bat, while being in mass storage mode.
If you've flashed the EFIESP with EDK2, just restart your phone, and press the Vol+ button (before the Windows Logo shows up).