[ONE Original Android Development] bigbadwolf_pig:
This is the guide I posted on the LOS 14.1 thread.

This is what I do all the time whenever I update to the latest LOS.

First install the latest TWRP(If you haven't already)

Make a nandroid backup(it will create an image of your current system and it includes apps, app data, messages, contacts, wallpaper, settings etc) using the backup function in TWRP(It will not backup mp3s, pics, videos and other stuff not related to the rom so back them up separately)

Nandroid backup is approximately 1.5GB to 3.5GB(really depends on how many your apps installed are)

***Optional, use titanium backup for your apps and sms/calls, so you won't have to install them again on the new rom. In my case I want them to be installed one by one from the playstore so I didn't use titanium back up.

***for backing up sms/calls I use SMS Backup & Restore from the playstore. But if your sms/calls are already backed up via google then just go that route.

Now plug your phone to your computer and just to be sure go to your phone's directory from the computer and locate the TWRP folder(that's where your nandroid backup is at) and copy it to your computer. Now even if something goes wrong at least your nandroid backup is backed up in your computer and you can just copy it back to your phone if you decide to go back to your old rom/image.

Now that the backup is set, download all of this to your computer:

This firmware:

Download 4.0.1.c7-00011 (2016) ***Note: If you already have a stable firmware you can skip this one.

https://androidfilehost.com/?fid=24389512218018449

Lineage OS:

LOS Bacon Weeklies/Nightlies

https://download.lineageos.org/bacon

Root:

Addonsu or Magisk(whichever you like)

Addonsu: https://download.lineageos.org/extras

Magisk: https://xdaforums.com/apps/magisk/beta-magisk-v13-0-0980cb6-t3618589

Lineage Debloat Script: (since I want to remove some stock apps from the LOS rom like the Snap Camera and other stuff I don't need)

https://xdaforums.com/oneplus-one/themes-apps/lineageos-debloat-script-t3546519

Pico gapps: (I just really want the bare minimum) or you can download any gapps package you want.

https://opengapps.org

Regarding the Firmware, I used that one on the link, because that's the one that's works for me. Now you can choose another firmware that works for you here:

https://xdaforums.com/oneplus-one/general/firmware-modem-oneplus-one-t3462214

Or you can just skip the firmware part if your current firmware is already working okay for you. In you case, you need to replace your firmware.

Don't use the ones that has H2O, oxygen, oxygen_update as I've read that some of these may cause some bootloops for LOS. I dunno. Try at your own risk.

[MENTION=4841620]Timmmmaaahh[/MENTION] made a comparison on the firmwares, check it out and see which one will work for you.

Firmware Comparison:

https://xdaforums.com/showpost.php?p=73925240&postcount=83

Regarding the Lineage Debloat Script, like I mentioned before, I really don't like the stock camera of LOS and other stuff so I just want to remove them. Now on the link there's gonna be instructions on how to remove them, you can remove them all or you can just choose what to remove. In my case I wanted the gallery2 app not removed so I didn't include that on the zip file I modified that I flashed on my OPO.

Okay all set. Your phone is still connected to your computer(make sure the android debugging is activated on your phone) Then reboot to recovery.

On the recovery:

Wiped Data

Wiped Cache

Wiped Dalvik Cache

Wiped System

Wiped Internal Storage(optional if you don't want to wipe it. It's gonna remove everything! Including the nandroid backup you made. So make sure that all your files and the nandroid backup is backed up on your computer)

Then go to your computer and copy all the things you downloaded to your phone and install them on queue.

Firmware(optional)

Lineage OS rom

addonsu or magisk

Gapps

Lineage Debloat Script

Reboot.

Now if you decide to remove the stock file explorer using the Lineage Debloat script then you will not have a file explorer to use. You can use ES File explorer from the playstore or You can use this if you want a root browser and file explorer all in one:

MiXplorer:

https://xdaforums.com/showthread.php?t=1523691

I'm not really sure how to install an apk file from TWRP so what I did was installed the ES file explorer first and installed the Mixplorer apk, then once the Mixplorer was installed I uninstalled the ES File explorer.

If you want to change the headphone volume(or other speaker, call volumes etc) if it's too low. You can increase them using the root browser from the mixplorer or any other root browser with an edit function. Just follow this instruction:

https://www.youtube.com/watch?v=viRMHSHLlrk

***Note, the path changed.

It should be: vendor>etc>mixer_paths.xml

Everything is all set.

It will probably take 30mins but I usually take about an hour because I double, triple check everything. Even though I written this instruction myself, I still read it whenever I flash a new rom because I tend to forget things from time to time. lol

Just remember nandroid backup on TWRP and then save the nandroid backup on your computer. Double, triple check it. :D

Good luck!﻿

Thanks!