[Fire HD 8 and HD 10 Original Android Development] vjunk:
Yes, but I had some trouble following the original instructions so put together some of my own. I only ran through these once so they are not perfect, but at least they provide more detail.

Start with the original instructions, then, when you get to this part:

Do the following:

Make sure ModemManager is disabled or uninstalled (as per the original instructions).

if the latest version of amonet-douglas is still 1.2, there is a bug in the code. To fix it:

a. Under amonet/bin, copy functions.inc to functions.inc.org.

b. Edit functions.inc. In the get_root() function, change the following:

while [ $ROOT -ne 0 ]; do
adb shell /data/local/tmp/mtk-su -c "/data/local/tmp/busybox mount /data/local/tmp/minisu.img /system/xbin/"
adb shell su -c id | grep uid=0\(root\)
ROOT=$?
[ $ROOT -ne 0 ] && sleep 1
done

to:

COUNTER=30
while [[ $ROOT -ne 0 && $COUNTER -gt 0 ]]; do
adb shell /data/local/tmp/mtk-su -c "/data/local/tmp/busybox mount /data/local/tmp/minisu.img /system/xbin/"
adb shell su -c id | grep uid=0\(root\)
ROOT=$?
[ $ROOT -ne 0 ] && sleep 1
echo Trying for $COUNTER more seconds...
((COUNTER--))
done

Download and extract "brick-douglas.zip".

Copy the extracted mtk-su (in one of the original steps) from the arm64 directory to the brick-douglas/amonet/bin directory.

Open another Terminal window on the computer and switch to brick-douglas/amonet. (We are not re-using the same window because we will come back to it.)

Run the following: sudo ./brick-9820.sh

Type "YES" and Enter to continue.

Power down the tablet and wait about 5 seconds.

Getting ready to let go of them quickly, power up the tablet into RECOVERY mode by holding down the power and volume downbuttons. AS SOON AS YOU SEE "RECOVERY" IN THE LOWER LEFT OR THE TEXT SCREEN WITH "Amazon system recovery <3e>", LET GO!

If you don't let go in time and the tablet reboots, catch it the next time.

Note, if you can't get into RECOVERY mode, it often helps to briefly let go of the power button and press it again as soon as you see the Amazon logo.

Get into FASTBOOT mode by using the volume buttons to move to "reboot to bootloader" and pressing the power button to select it.

The script should continue as soon as the tablet is in FASTBOOT mode. If it doesn't and stays at "< waiting for any device >", press Ctrl+C to terminate the script, then re-run it.

The script may pause at "Writing 'brick'...". Watch the tablet and, when it says "All good...", wait a few seconds, unplug the USB cable from the tablet, and power it off by holding down the powerbutton until the screen turns off.

Note, the script may say it FAILED. As long as the tablet said "All good...", the script succeeded.

Switch back to the other Terminal window, the one in the "amonet-douglas-v1.2/amonet" directory.

If you closed it or re-used it for the bricking procedure, switch back to amonet-douglas-v1.2/amonet.

You can close the "brick" window or leave it open for now in case we need to use it again.

Next, run the following in Terminal: sudo ./bootrom-step.sh

DO NOT RUN THE ONE WITH "minimal" IN THE NAME!

Plug the USB cable back into the tablet. It should power itself up but you will not be able to tell since the screen will remain black.

If the script does not show "* * * Press Enter to continue * * *", you may need to momentarily press the power button.

Wait about 15 seconds. If the script still does not show "* * * Press Enter to continue * * *", make sure the USB cable is firmly plugged into the tablet and computer. For best results, use the cable that came with the tablet.

Hold down the power button for at least 50 seconds as a last resort! If you STILL cannot get it to work, stop the script with Ctrl+C and restart it. REMEMBER, THE TABLET'S SCREEN SHOULD REMAIN BLACK as the script re-flashes it.

You can see if the tablet is connected to the computer by running the following in Terminal: sudo lsusb

It should show something like "MediaTek Inc. MT65xx Preloader", "MediaTek Inc. MT6227 phone", "Lab126, Inc. Android", or "Lab126, Inc. Fire". The correct one that it should be in is "MediaTek Inc. MT6227 phone".

Worst case scenario, there is a recovery method that should work that involves opening the tablet. See Fire hd8 2017 root, debrick for details.

Press Enter in Terminal to continue.

The script should end with "Force fastboot", "Flash preloader header", and "Reboot" and the tablet should reboot into HACKED FASTBOOT mode. However, it's difficult to tell SINCE THE SCREEN WILL REMAIN BLACK!

If you are sure the tablet did not reboot, try pressing the power button for about 15 seconds, waiting a few seconds, then pressing it again for about 15 seconds. If that doesn't work, repeat the same process but hold the power button down for at least 45 seconds both times.

Note, if the tablet is powered up, running "lsusb" in the Terminal window should show one of these: "MediaTek Inc. MT65xx Preloader", "MediaTek Inc. MT6227 phone", "Lab126, Inc. Android", or "Lab126, Inc. Fire".

If it powers up to FASTBOOT mode, return to Step 6. Note, if sudo ./brick-9820.sh didn't work properly, you can try sudo ./brick.sh instead. If you do, keep in mind that you should run sudo ./bootrom-step-minimal.sh instead of sudo ./bootrom-step.sh in Step 14.

While in HACKED FASTBOOT mode (still a black screen), run the following: sudo ./fastboot-step.sh

When the script is done, it will say "Your device will now reboot into TWRP." and the tablet will reboot into TWRP.

Once the tablet reboots into TWRP, you are now past Step 4 in the original instructions and can proceed to install Magisk and/or follow the [ROM][UNOFFICIAL][10] LineageOS 17.1 for Amazon Fire HD8 7/6th gen (douglas/giza) thread. Note, if installing the "2023-05-20" build from that thread, before trying to flash OpenGapps, delete the app-sizes.txt file from the ZIP file; otherwise it won't fit. You can also install microG or BitGapps core instead of OpenGapps.

Good luck!