[Kindle Fire HDX 7" & 8.9" Android Development] DB126:
Unlocking the bootloader is a bit challenging on HDX devices. Good news is nothing will break if you get it wrong; the unlock code simply won't work. Your personal sanity may be another matter as it can be a long, frustrating experience for you anyone within earshot (lol). Which is why I'll outline a couple paths one of which does not required unlocking. Btw, you linked to my preferred resource for bootloader unlocking. There are other resources but I have found "post #67" by [MENTION=3510401]ceyo14[/MENTION] to be one of the most helpful.

First, make a backup of your current system from twrp:

- power down device

- presss power button and vol+up (volume up) at the same time. Release the power button when the grey logo appears followed by vol+up a few seconds later.

- select 'backup' from twrp. Accept default partition selections (Data, System, Boot) and location (internal storage). Swipe to begin backup.

- when complete restart your kindle and copy the backup file to another system (it will be located under sdcard/twrp). This is best accomplished via tether but can be done over wifi in a pinch. Let me know if you need help with this step.

- you can delete the backup file from you device is space is tight. Make sure you have at least a couple GB of storage remaining for the next step.

At this point you have two Nexus options:

1) Flash v2.0.5 which is Jelly Bean based but works with a locked bootloader.

2) Flash v4.0.x which is KitKat based but requires the bootloader to be unlocked (which will take some time/effort).

I suggest you start with option #1. v2.0.5 is a very nice rom, feature complete and offers full access to the Google Play. Immediate gratification! You can work on unlocking the bootloader in parallel as time permits and subsequently upgrade to v4 at the time of your choosing. Just remember to make a twrp backup from time-to-time in case you need to restore an earlier version.

To flash Nexus v2.0.5:

- download the rom, gaps and dolby zips from this post

- follow the instructions under 'fresh installation' in the same post. All flashing takes place from within twrp. Click 'install' and then navigate to the directory that contains the downloads from the previous step. After adding the first zip you will be presented with an option to add others. I tend to do each one individually but you can combine and flash all in one step. Once complete do a 'factory reset' which can be found under the twrp 'wipe' option. Reboot and enjoy!

Of course there are a few cautions:

- make sure you grab the correct files for your device (Apollo)

- after downloading you can verify MD5s from FireOS using the Hash Droid tool installed earlier. Nothing can ruin your day quicker than flashing a damaged/corrupt image.

- there is a small risk associated with every flash. Double check all steps before executing. If a flash goes bad you can usually recover from backup (make those backups!)

- never flash to the boot or recovery partitions unless explicitly instructed to do so!

- an unlocked bootloader is the ultimate protection against a bad flash or other system catastrophe. Recovery from this type of calamity is not easy but can only be done via Fastboot which requires an unlocked bootloader on HDX devices. That is why you will see the recommendation to unlock your bootloader posted by ggow and other developers. That said - flashing Nexus 2.0.5 is not particularly dangerous provided you follow instructions.

Good luck!