My version of the
feedthebeast AUR package is a simple modification. It simply forces the use of OpenJDK 8's JRE as the default if it's installed, even if it's not the default. This is beneficial because FTB's authlib fails to load under Java 12, and Minecraft performs like shit on most non-8 JREs. Using this version you don't have to change your default JRE to 8 to get Minecraft working correctly as long as it's installed.
As an Arch user you probably have already heard this a million times, but to reiterate: DON'T USE AUR PACKAGES UNLESS YOU KNOW WHAT YOU'RE DOING AND YOU TRUST THE PACKAGE DISTRIBUTOR!
Clone this repository
cd into it
About the recommended makepkg arguments:
-iautomatically builds and installs the package
-suses pacman to fetch any required dependencies for building and installing
-rremoves build dependencies after the build is completed (to keep your system nice and clean)
-fforces a rebuild in case it's already built
Bye. That's it. I have nothing else for you.