Paste the following into your /etc/pacman.conf, preferably near the top of it.
[lucifers-linux]
SigLevel = PackageRequired
Server = https://sourceforge.net/projects/luciferslinux/files/repository/$arch
[lucifers-linux-any]
SigLevel = PackageRequired
Server = https://sourceforge.net/projects/luciferslinux/files/repository/any
Sync your package database so pacman is able to retrieve packages from there.
pacman -Sy
Install away!
## In the case of an existing installation
pacman -S <packagename>
# or
pacman -S lucifers-linux/<packagename>
If you are installing a new installation of Lucifer's Linux, please start by following the Arch Linux Install Guide. Once you have finished following the installation instructions, you can reboot into the new system and install further packages.