Steps to flash microG LineageOS on Samsung Galaxy S8+

Prerequisites

A Samsung Galaxy S8+ (make sure it says SM-G955F in the "About Device" settings)
A computer (preferably Linux) with:
- an USB slot
- adb installed
- heimdall-flash installed (on Windows you can use Odin)
A micro-USB cable which can transfer data

Preparations

download latest TWRP image from https://eu.dl.twrp.me/dream2lte/

download latest lineageOS unofficial build from https://mega.nz/folder/NYpgFQCY#VYa2sKGXY83vEjOHZ-rIZg

Unlock the Bootloader / OEM

Attach phone to PC. Use a good USB-C cable; make sure that you can use it to copy data.

Enable developer options

Enable USB debugging in developer options

Allow USB debugging access for your PC

Allow OEM unlock

Flash Recovery

*Right now, flashing recovery only works with Odin; Odin only runs on Windows. Guide how to flash TWRP with Odin: https://www.cyanogenmods.org/forums/topic/install-twrp-recovery-samsung-android-using-odin/

If Heimdall works:*

Power off the your device and connect the USB adapter to the computer (but not to the device, yet).

Boot into download mode:

With the device powered off, hold Volume Down + Home + Power.

Accept the disclaimer, then insert the USB cable into the device.

heimdall flash --RECOVERY twrp-*dream2lte.img

Now we don't want to boot into the system, we want into the new recovery directly - so turn off the phone with the power button, and then boot into recovery by holding the power button and volume up for a few seconds. Release them only when you see the blue "Recovery" writing in the top left of the screen.

Then go to Wipe + Advanced Wipe to wipe the following partitions:

System
Data
Cache
Dalvik

We also had to "Format Data", because the data partition could not be mounted first.

Then go to Advanced/ADB Sideload, swipe to the right, and execute adb sideload lineage-xxx.zip on your computer.

When it shows something like Total xfer: 1.00x or adb: failed to read command: Success on the PC, and Exit code: 1.000000 in TWRP, it has suceeded.

Unfortunately addonsu doesn't exist for lineageOS 17, so we need to find another rooting tool, e.g. Magisk. This would also be flashed at this point.

Reboot to start LineageOS!

Finish Installation

Go through the setup screen and choose what you want. You can be as restrictive as you want. Make sure to choose a Wi-Fi, so you can proceed with the next steps; you can also do this at any later time.

You should also choose a screen lock pin. Patterns are not very secure (you can often see them by holding the phone into the light at the right angle). Passwords are a bit inconvenient (but most secure). Do not use a fingerprint sensor, they are really insecure.

Device Encryption

Charge the phone to 100%.

Go to the Security Settings and encrypt the phone. During the process, it reboots once and asks you for the device PIN. It can take a while, but not soo long actually.

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