Now that 6.2.1 is here ruining our party, it's time to package this up for anyone to use.
As useful as they are now, fastboot cables may not work in the future. They rely on the bootloader to work, and it's possible that a future OTA could disable fastboot.
For the adventurous. This is mostly untested by me, and runs a good risk of fuglifying or permanently breaking your Kindle. If you haven't voided your warranty yet, this will.
Install TWRP over USB
0) You will need a PC with Linux and working adb, the .zip attached to this post, and the installer version of TWRP[/URL]. Also a microUSB cable. And something to short the contact like tweezers or a bent paperclip.
1) Unpack the rekindle .zip and copy TWRP into the directory it creates. Change directories into rekindle/
2) Open a terminal and sudo or su to root. It's easier that way.
3) Unplug the USB or AC adapter if it's plugged into the KF. But have the USB cable's A end plugged into the PC. This is very important.
4) Turn the power completely off. Do a shutdown if actually running Android, or hold the power button until there's no LED or backlight.
5) Pry open the back cover. The iFixit teardown (Google it) gives some ideas on how to do it, but be really careful because it's easy to snap the tabs along the long sides.
6) The power must still be off. If you accidentally powered it back on, turn it back off.
./usbboot aboot.bin u-boot.bin; ./fastboot boot twrp-blaze-2.0.0RC0.img
8) Short the point shown here to the metal frame around the CPU area using your paperclip or whatever. While keeping it shorted, plug in the USB cable. This will power up the Fire with the CPU in USB boot mode.
9) If it works, you'll see some text fly by in the terminal, and you'll see the yellow triangle hopefully followed by TWRP starting up. You can follow the instructions in the TWRP post on completing the install.
***This likely won't work***
There are reports of problems booting after applying this bootimage. Try the TWRP install above and one of the root update.zips instead.
0) You must already be running 6.2.1. Otherwise try a safer method.
1) Get the rooted 6.2.1 bootroot .img from here..
2) Follow the procedure to install TWRP above, but stop before step 7 (the usbboot command). You can skip downloading TWRP.
3) Run this command instead:
./usbboot aboot.bin u-boot.bin; ./fastboot flash boot 6_2_1rootboot.img
5) Hold down the power button until it powers off (~15 seconds), and press it again to power it back up.
Windows & OSX support
Currently usbboot is built for Linux only. I had experimented with building it against libusb for Windows and OSX. Unfortunately the window to make the connection before the CPU resets again is about 2 seconds, and Windows takes a lot longer than that to enumerate new USB devices. I don't know about OSX, but I guess a libusb version (usbboot's USB code is Linux-centric) would work fine. Github is here if you want to take a crack at porting it.
Successes? Failures? Smoking hole in the ground that used to be your Kindle? Post here.