That to be said, let's start: i am currently running the latest official nightly build on my d855 and it took me a while to get there. I couldn't find any (real) step-by-step-guide, where i would not have to google any errors or anything else.
Everything you find here is NOT my own knowledge, it is just compilation of sources, that i found on the internet.
Android 6.0: if you are on 6.0 and you always updated so you're on the latest security-patch-level the one-click-root-method won't work. I recommend downgrading your phone, which will be explained in the following.
Android 5.0: if you're on 5.0 you need to upgrade to 6.0 anyways, bc you need a different baseband-version to install LineageOS. This will also be explained in the following.
1. Make a backup of your phone
this is recommended, although i didn't do this, this is why i am not gonna explain it one here. I just saved my pics and contacts and did a factory reset, restarted my phone, did all the "first time turned on stuff" and connected to my wifi.
Note: Make sure your phone is charged to 60% at least
2. Download the following
a. Download LG G3 Drivers 4.0.4 and install them (i believe there are more recent ones, but this worked for me)
b. Download LG UP
- Download: LG UP Ver. 1.14: LGUP_Install_Ver_1_14_3.msi and LGUP_8974_DLL_Ver_0_5.MSI
- Make sure to install the DLL first and then the other .msi
SOURCE: Thanks to hyelton
c. Download the proper KDZ for you phone.
Note: I got this infos from a german site.
This is the 30C KDZ for the 32gb AND 16gb version: Download here
SOURCE: Thanks to mr bo
3. Downgrade/Upgrade your android-version
1. Turn off your phone.
2. Run LG UP (the tool you just installed) and press okay when you see the error message, that no phone is connected.
3. Press "Volume Up" on your phone and HOLD it, connect your phone to you computer via USB 2.0 PORT! This is important, bc connecting via 3.0 won't let LG UP recognize your phone.
4. Your phone is now in download-mode and LG UP should have recognized your phone.
5. Press SELECT, click on UPGRADE, then click on FILE PATH, choose you prefered .KDZ.
6. i recommend refurbishing your phone (This will wipe your entire system and make a clean install).
CLOSE LG UP and start it back up. The .KDZ should still be there. Now select REFURBISH and click on START. Wait until 100% completed and you phone has booted up completely.
Source and further information: Hail hyelton!
Also here is a youtube video: Thanks to kiaTech
4. Root your phone
1. Download the latest kingroot.apk from here.
2. Connect your phone (turned on, after all the "first time turned on stuff") and move the .apk to your Download-folder.
3. Open the (stock) filemanager on your phone and navigate to Downloads.
4. Install the kingroot.apk (enable third party .apk, when asked)
5. Open the app and root your phone (it's simple, believe me).
6. Your phone is now rooted (leave it turned on, DON'T restart)
5. Install TWRP and update to the latest version
1. Download this pack and unzip it.
2. Move the AutoRecMMD855.apk to your phone and install it just like kingroot.apk before.
3. Open the app and follow the instructions.
4. You now got TWRP on your phone.
SOURCE for #4 and #5 till here: Thanks to tute123456
5. Boot into recovery mode (either with the app itself i think or manually. If you have to do it manually. Turn of your phone. Press POWER and VOLUME DOWN at the same time until the screen shows soemthing. Release and immideatly press again. Your phne will ask you if you want to FACTORY RESET. Press YES and YES. If everything is working properly, TWRP should start up).
6. Connect your phone and download the latest TWRP.img you. In example here's the european one Latest LG G3 TWRP EUROPE
7. I assume you never disconnected your phone since moving kingroot.apk to it. So now you just move the TWRP.img to your Download folder.
8. Choose INSTALL on your phone and search for the TWRP.img. Select RECOVERY and flash it.
9. Choose REBOOT and then RECOVERY.
10. Your phone will boot into TWRP again. It should be updated to the latest version 3.0.2-0.
SOURCE: Thanks to everyone who contributed
6. Flash LineageOS and install GApps
1. Download the latest version of LineageOS (nightly) from here
Optional: make a backup of your phone using TWRP and safe it on your pc or internal SD Card, bc in the next step you will wipe your entire system, to get a clean installation.
2. Press WIPE in TWRP and select ADVANCED WIPE. Select everything but "Micro SD Card" and perform the wipe. This will wipe your entire system!
3. Move the .zip file to the Download folder on your phone and press INSTALL in TWRP. Search for the .zip and flash it.
Note: you'll see two red error messages. This is normal!
4. Wipe cache and dalvik as recommended.
5. Download your desired Open GApps package from here. Use "ARM" and "7.1", i recommend the "mini"-package.
6. Move the downloaded file to your Download folder on your phone and flash it just like you did the LineageOS file.
7. Wipe cache and dalvik.
8. In the main menu of TWRP choose REBOOT and then SYSTEM.
9. It will take a while to boot up, don't worry, just leave it there.
10. You succesfully installed the latest LineageOS build on your phone - you can now unplug your phone from your PC also
SOURCE: LineageOS official wiki