[GUIDE/HOW-TO] Building LineageOS for an Unsupported Device

Foreword:
This is my own notes I created to build LOS for my device (SM-T713 or gts28vewifi). After reading this, I encourage you to create your own notes as it will help you better understand the build process.

I followed the official guide here with additional reading here since we are building this for a device no longer officially supported by LOS.

Requirements:
A fast CPU. How fast? All depends on how long you can wait for the build.
At least 16Gb RAM is a must.
Around 300Gb 50Gb disk space since at least 100Gb of it will be devoted to "ccache".
*nix system, mine is on Ubuntu so the commands below are Debian-based.

Step 1: Setup the environment.
sudo apt install -y bc bison build-essential ccache curl flex g++-multilib gcc-multilib git gnupg gperf imagemagick lib32ncurses5-dev lib32readline-dev lib32z1-dev liblz4-tool libncurses5 libncurses5-dev libsdl1.2-dev libssl-dev libxml2 libxml2-utils lzop pngcrush rsync schedtool squashfs-tools xsltproc zip zlib1g-dev python-is-python3

mkdir -p ~/bin
mkdir -p ~/android/lineage

curl https://storage.googleapis.com/git-repo-downloads/repo > ~/bin/repo
chmod a+x ~/bin/repo

git config --global user.email "[email protected]"
git config --global user.name "Your Name"

cd ~/android/lineage
repo init -u https://github.com/LineageOS/android.git -b lineage-17.1

Step 2: Get the device manifest file. ...and save to ~/android/lineage/.repo/local_manifests/gts28vewifi.xml

Then do:
repo sync

Step 3: Turn on caching to speed up build. Note that I used 300Gb instead of 100Gb (just personal preference).
mkdir ~/ccache
export CCACHE_DIR=~/ccache
export USE_CCACHE=1
export CCACHE_EXEC=/usr/bin/ccache
ccache -M 300G 50G

Step 4: Select Build.
source build/envsetup.sh
lunch
select lineageos_gts28vewifi-userdebug (whatever number it's identified under)

Step 5: Prepare the output:
make clean-apache-xml
make clean
make apache-xml
make ims-common

Step 6: Build it!
brunch gts28vewifi

Now go have a few coffees and come back later.

Epilogue:
I'm sure there are many different ways to build this but this method works for me. Just want to get the knowledge out so you can find a way that works for you.