[Kindle Fire HDX 7" & 8.9" Q&A, Help & Troubleshoot] DB126:
Super! Final step is to install Nexus. Grab the Thor (nickname for 3rd gen HDX 7) installation zip from here. GAaps and SuperSU are built in so it's the only file you will need. Transfer that to a safe location on your device, preferably in a folder you create directly under sdcard. It is important this file (or it's successors) remain on your device at all times for emergency recovery. Putting it in the suggested location reduces the chance of accidental removal when wiping the (user) data partition. You can also place the folder directly under root (/) but be aware some file managers and tethers restrict access to high level directories.

Important: Unlike TWRP Safestrap v4.0.1 recovery has no ability to communicate with the outside world. Not by tether, wifi, bluetooth, adb, etc. It is fully isolated environment. If your rom becomes unstable (rare...but it does happen) your only recovery is using a previously created backup or a flashable rom image (as discussed above) resident on the device.

Boot into Safestrap recovery. Use the File Manager (under advanced) to confirm the existence/location of the Nexus rom zip. Return to the home screen and then access the wipe menu. Use the slider at the bottom to perform a factory reset which wipes the data, cache and dalvik partitions. Note this is different that a stock 'factory reset' which was discussed in a previous post. Also note it is impossible to wipe the system partition in Safestrap v4 even if selected in the advanced section. I won't get into the reasons; just know it isn't necessary for the upcoming operation.

Return to the home screen and select install. Navigate to the folder containing the Nexus rom, select it and then and then swipe the slider at the bottom to install (flash) the zip which will overwrite FireOS. When complete (assuming no errors) reboot your tablet and select continue to fire up Nexus. First boot make take some time as caches are populated. However, anything over 5 min indicates a problem. Once Nexus starts I assume you know where to go from there. You can safely enable WiFi, register with Google and perform all other requested initialization steps.

A few final footnotes:

- Nexus is pretty bare bones; not even a browser in included in the base build. Head over to the Play Store to grab what you need

- While Google's stock launcher is nice I suggest replacing it with Nova which offers a superior AOSP experience along with robust customization

- Xposed Framework and associated modules offers some excellent additions to this rom if you wish to customize further (advanced topic)

- Once you are confident Nexus is stable you can delete the huge FireOS backup created in an earlier step. Either via ESFE or from Safestrap

You may be aware Safestrap supports multi-boot through the use of user created slots. Unfortunately, they do not work with this device and can lead to unrecoverable bricks. You'll see various cautions in the Safestrap, Nexus and CM threads. Short version: do not create or use secondary slots.

Aaron - Feel free to post any questions/concerns/comments. Someone from the HDX community will likely respond within 24-48 hours. On a personal note it's been awhile since I did a long hand 'step-by-step' which is a refreshing change of pace from the usual bullets. I appreciate your patience as I dusted off various cob webs and hope you make the most out of your newly liberated device.