On my phone so I can't give links. But you need safestrap. It's a bootstrap that will essentially allow your phone to boot into recovery even with the locked bootloader.
Safestrap is unique because it does not touch your primary system. You can back it up, but you flash everything to a "safe" system. It is ideal because no matter what you doing to your safe system, your primary will always be there. Even if you flash something that you know will brick your phone, all that will happen is that you won't be able to boot into your safe system. You can still load up your custom recovery and switch back to your primary system where you can download a different ROM.
Sent from my DROID3 using xda premium
Droid 3 with Gummy 1.2.0 (Unthemed) [Retired]
Galaxy Nexus with CNA 3.6.6 and Franco's Kernel [Retired]
Galaxy S4 with KitKang 11-20-13 and Stock Kernel