[Kindle Fire HDX 7" & 8.9" Q&A, Help & Troubleshoot] snorklm:
From many many different threads and frankly confusing instructions I think I have puzzled together the following solid guideline for my kindle fire hdx from basic root to custom rom. Currently my firmware is 13.3.1.0.

Could someone take a look at this and tell me if I got this right or if I need to do stuff differently (or could do them better)? Don't assume that something is obvious, if it isn't written down, don't be shy to mention it. I would be ever so grateful :).

Edit: I'm updating my experience as I move along the list and get more info just in case this might be useful to someone who is just as lost as I was in the beginning ;)

Obligatory disclaimer: this is just a step-by-step guideline that worked for me. I'm not responsible if you chose to follow any of these steps and damage or brick your device.

Guideline:

-Download firmware version 3.2.3.2 and upgrade directly from any of the previous version by putting the file into the internal storage of the kindle, going to settings>device>system update on the kindle and installing the update from there.

-Root with Towelroot in the HDX toolkit downloaded from here (Edit: I got various error messages at this step but got it to work in the end. The 3rd post here shows what I did to get it working after getting error message 2 and 3 at various times)

-block OTA also with the tool in the HDX toolkit (is there a way to safely check that this worked? Edit: It worked )

-connect to wifi and install Flashify from the playstore (Edit: I ended up installing File Expert and flashify on another android device, backing up the flashify apk and sideloading it with ES File Explorer from the amazon app store to the kindle, I guess I could have also download the apk somewhere but I have trust issues.)

-download the img of TWRP 2.8.5.0. and put it in internal storage of kindle (had no problems)

-use flashify to save original stock recovery (had no problems)

-Use flashify to flash TWRP (had no problems)

- Boot into recovery mode via flashify to see if it worked (had no problems)

Now either A)

-download CM11 image and gapps and put the zip files on internal storage

-Edit: disconnect Kindle from USB afterwards ;)

- go into TWRP recovery mode (by powering off kindle and then pressing power and volume at the same time)

- make backup of original stockrom under backups and save copy on your Computer

- wipe system, data, cache and dalvik cache under wipe/advanced (all except internal)

- flash cm11 image and gapps (under install and then find them in the files)

- reboot

-Edit: I had the wifi not connecting problem that some people have with CM11 but a reboot took care of it.

-Edit 2: I couldn't get the kindle app to work on CM11 (it wouldn't synch my books, the device wasn't recognized by amazon and it would crash every time I logged in.) so I ended up replacing it with the Nexus rom. If the chance of this happening doesn't bother you then CM11 seems to still be a good choice.

or B)

- download HDX thor nexus rom and gapps and put the zip files on internal storage

- go into TWRP recovery mode (by powering off kindle and then pressing power and volume at the same time)

- make backup of original stockrom under backups and save copy on your Computer

- wipe system, data, cache and dalvik cache under wipe/advanced (all except internal)

- Edit: disconnect Kindle from USB afterwards ;)

- flash nexus rom and gapps (under install and then find them in the files)

- reboot

- Edit: I'm very happy with it, it works flawlessly. Since it's based on Jelly Bean I may try replacing it with another rom sometime in the future, but I'm in no hurry :)