[Xiaomi Redmi 6 Questions & Answers] yellowfoxstaris:
Okay, I managed to get my phone back in a working order.

I will sum up here what happened and what I did for future generations.

Symptom: Phone is Xiaomi Redmi 6. (this tutorial might work for another models, but I will presume that you have this phone) Phone has black screen and vibrates for cca 20 seconds then stops and vibrates again.

Background: If you tried flashing your phone and you got into this state, don't panik, you can still save it!

Prerequisites:

Flashing of your phone is unlocked.

Your computer has ADB and Fastboot Drivers.

If you broke your phone via flashing, you most likely have those two checked. If not, please find another tutorials to get into this state.

I will now write tutorial, each step is labeled by a number, if needed I will write some short comment for each step.

1. Follow this tutorial and open back of your phone.

2. Disconnect your battery. (You don't need to unglue your battery in order to do so!)

(Now your phone shouldn't vibrate and should be turned off, take a breather, you don't need to listen to the vibrations anymore, from this state, you can access fastboot, but we need to prepare a few more tools. In theory you can close phone now, but I would recommend to do it after the phone is in fully working order.)

3. Download Xiaomi ADB/Fastboot Tools.

GitHub - Szaki/XiaomiADBFastbootTools: A simple tool for managing Xiaomi devices on desktop using ADB and Fastboot

A simple tool for managing Xiaomi devices on desktop using ADB and Fastboot - GitHub - Szaki/XiaomiADBFastbootTools: A simple tool for managing Xiaomi devices on desktop using ADB and Fastboot

github.com

4. Follow the tutorial on the github to install Java JRE.

GitHub - Szaki/XiaomiADBFastbootTools: A simple tool for managing Xiaomi devices on desktop using ADB and Fastboot

A simple tool for managing Xiaomi devices on desktop using ADB and Fastboot - GitHub - Szaki/XiaomiADBFastbootTools: A simple tool for managing Xiaomi devices on desktop using ADB and Fastboot

github.com

5. On your phone hold VOLUME DOWN + POWER. (Your phone should boot to Fastboot screen.)

6. Turn on Xiaomi ADB/Fastboot Tools and connect phone to computer.

7. Verify which architecture you have and download appropriate official ROM. (You can download it from Xiaomi Tools!)

8. Unpack the downloaded ROM (I had to unpack it twice).

9. Click select ROM folder in Xiaomi tools and select folder which you unpacked. (It should contain sh files, bat files, py files and folder "images")

10. Flash ROM.

Now if everything goes well, congratulations, you may pat yourself on the back, your phone should now work as it worked before. Whatever you did before, please be more careful this time.

I will now describe what I continued to do.

Follow optional steps ONLY if you want to install LineageOS.

I am including following steps only because I want to complete steps for what I did. If you wanted only to recover your phone, you are good to go.

I originally followed this tutorial:

https://xdaforums.com/t/rom-unofficial-lineage-os-17-1.4252115/

OPTIONAL STEPS:

(you have been warned)

1. Download OriginalROM

Download Redmi 6 (cereus) V11.0.4.0.PCGMIXM MIUI 11 Stable Full Rom | XiaomiROM.com

The Stable update V11.0.4.0.PCGMIXM has been officially released for Redmi 6 (codename is cereus). Before updating, please make sure you have …

xiaomirom.com

2. Download LineageOS from previously stated tutorial.

3. Download TWRP.

Download TWRP for cereus

Download TWRP Open Recovery for cereus

eu.dl.twrp.me

4. Download Magisk.

Release Magisk v21.4 · topjohnwu/Magisk

Update: v21.4 adds more regression hot fixes. Happy 2021! v21.3 adds a workaround for devices with buggy F2FS Linux kernel drivers. This F2FS bug may cause bootloops on many devices. Checkout the f...

github.com

5. Download Open GApps.

The Open GApps Project

OpenGApps.org offers information and pre-built packages of The Open GApps Project. The Open GApps Project is an open-source effort to script the automatic generation of up-to-date Google Apps packages. All Android versions and platforms supported.

opengapps.org

(so far you basically followed previously mentioned tutorial)

(from now on, I had to deviate from tutorial a little bit)

6. Depending if you are on the Windows/Linux/Else edit appropriate files in Original Rom which are named (Flash All, etc) so they don't contain "fastboot reboot" (this will be found to the end of file).

6. Via Xiaomi tools, flash Original ROM.

7. Reboot from Fastboot to Fastboot.

8. Flash TWRP.

9. Boot into recovery.

10. Connect phone and copy LinageOS, Magisk and GApps.

11. Flash LinageOS, Magisk and GApps (I did it in this order, I don't know, if it makes difference.)

12. Enjoy your phone.

For me, if I flashed rom without editing the files, the phone booted and user data were created, and then TWRP was unable to flash them. You can probably get around this via wiping out user data or another way.

I hope that this helped you and good luck with flashing!

Thanks to:

pl1992aw and Austinredstoner for getting me out of this mess

EinTim32 for creating the original tutorial

Szaki for Xiaomi tools

TWRP, Magisk and LineageOS developers for creating amazing software!