[Kindle Fire HDX 7" & 8.9" Q&A, Help & Troubleshoot] DB126:
What follows is a 'mini-guide' for adding Safestrap v3 and Nexus v1.0.1 (Safestrap varient of Nexus HDX v2.0.5) to a stable, rooted HDX running FireOS xx.3.2.6 with OTA blocked. Any other use may yield unpredictable results. Note Safestrap v3 and Nexus v1/v2 are older builds. Neither is actively supported but continue to function well. There are reasons for going this route which can be derived by viewing previous posts. In most cases it is advisable to consider newer variants of these components which continue to be actively supported and enhanced.

Important: Take note of the date of this post and and any recent edits. If under 72 hours you may want to wait as there are likely small errors/omissions that did not get caught in proofreading. Each and every major step contains some level of risk - including a full/hard/unrecoverable brick of your device. Proceed at your own risk. I take no responsibility for outcomes, good or bad, even if you followed the guide exactly as written. If uncertain just walk away.

Part 1 - Install Safestrap v3

- verify device satisfies requirements outlined in opening paragraph

- understand terms such as 'adb', 'apollo', 'recovery', 'rom', 'twrp' and 'thor'

- enable adb support (settings -> device -> developer options)

- enable sideloading (settings -> applications -> unknown sources)

- download and install Safestrap v3.75 b02 from here

- launch Safestrap (from FireOS), accept disclaimer and install Safestrap Recovery

- reboot Safestrap recovery. Device will restart to a facsimile of TWRP recovery. Have a look around but DON'T CHANGE ANYTHING

- press 'reboot' button from within Safestrap Recovery followed by 'system' button

- note the new Safestrap spash screen with robot caricature. Just let it time out (10 sec)

- FireOS should now boot. Congratulations! You have successfully installed Safestrap v3

Part 2 - Backup current FireOS environment

- power down device and restart

- when Safestrap splash screen appears tap 'Recovery' (lower left corner)

- when Safestrap starts insure 'ROM Slot: stock' is shown in red at top of screen

- tap 'Backup'. Tap 'Storage...' bar (black stripe) toward bottom of screen, select 'Internal Storage', then tap 'OK'

- insure 'Data' and 'System' partitions are selected. Then 'Swipe to Back Up'

- when complete tap 'Reboot System' and return to FireOS

- congratulations! You have backed up FireOS v3.2.6 (or whatever happened to be in the stock slot)

- (optional): Best practice is to copy backup folder to another machine for safe keeping. This requires tethering your HDX via USB cable typically to a Windows PC. From Windows you will find the a backup folder located under 'TWRP' while viewing your Kindle's Internal Storage. Copy the entire backup folder to the host PC. It can then be deleted from your device if more space is needed (you should have at least 5GB free for Nexus installation and associated apps/data). Additional tethering details are beyond the scope of this guide.

Part 3 - Create a new Safestrap 'slot' (container) for Nexus

- note: FireOS will remain installed; however all apps, files and settings will be isolated from other 'slots' (ROMs)

- power down device and restart

- when Safestrap splash screen appears tap 'Recovery' (lower left corner)

- from Safestrap recovery tap 'Boot Options'

- tap 'ROM-Slot-1'. Leave defaults as shown. Tap ' Create Rom Slot'. Patience is a virtue ... (~5 min)

- when complete verify 'Rom-Slot-1' is now selected. ALWAYS TAKE NOTE WHICH SLOT IS SELECTED

- tap 'Stock ROM' followed by 'Activate'. Verify stock selection was successful (top of screen)

- return to main screen and then reboot FireOS. Congratulations! You have now created a slot for Nexus

Part 4 - Flash Nexus v1.0.1

- download Nexus v1.0.1, gapps v1.2 and Dolby Digital v1.2 from here

- (recommended): for reference obtain a hard copy of the instructions contained in the above post

- move above files to a folder under 'internal storage' on your device (suggest 'downloads')

- (recommended): verify MD5 of downloaded files on your device. Technique for doing this is beyond scope of this guide

- access Safestrap recovery environment

- tap 'boot options' followed by 'ROM-Slot-1'. Tap 'Activate'

- IMPORTANT: verify ROM-Slot-1 is selected (look for notation at top of screen)

- tap home/house icon to return to Safestrap Recovery main menu

- tap 'install' (um...did I mention verifying you are on 'ROM-Slot-1'??)

- navigate to the folder containing Nexus ROM and related downloads (zips)

- select Nexus from from list of zips

- tap 'Add More Zips' and select 'gapps v1.2'

- (optional): tap 'Add More Zips' and select 'Dolby Digital v1.2

- tap 'Swipe to Confirm Flash'

- assuming flash was successful tap 'Wipe cache/dalvik' button and 'Swipe to Wipe'

- tap 'Back' followed by 'Home'

- Reboot system; note Safestrap slash screen will indicate secondary slot is active

- Allow reboot to progress normally. Congratulations! You have flashed Nexus v1.0.1

Notes and Troubleshooting

- If Nexus fails to boot long press power button (10-20 sec) and release. At Safestrap splash screen select 'Recovery'. From Safestrap Recovery select 'Boot Options' followed by 'Stock ROM' and 'Activate'. Press the house/home button (lower left corner) then reboot system. FireOS should then start.

- Initializing and using Nexus is beyond the scope of this guide. Please refer to the HDX forums for Q&A. If you can't find the answer there post a question.

- If you screw-up and flashed Nexus to the Stock ROM slot there are ways to recover provided Safestrap itself is not damaged. Details are (you guessed it...) beyond the scope of this guide.