To install in terminal:
first refresh repos:
sudo pacman -Syu
The generic install command is
sudo pacman -S packagename
Octopi is the Qt GUI for Pacman and is normally the software manager in the Arch-based-or-related systems that use KDE. Those that use Xfce or other GTK desktops will probably have Pamac, which is a GTK GUI for Pacman. It is similar to Synaptic.
Octopi is able to install from already built packages (.pkg.tar.xz) but not tarball archives (.tar.xz).
Before searching for a program in Octopi be sure to refresh repos.
When the package is absent in repos, it can be built as follows:
Download the tarball package from AUR to (e.g.) ~/Build folder and extract it there, then go in terminal do
- in order to build without installing
makepkg -s(that will create an archive package .pkg.tar.xz in the package_name folder that can be installed with Octopi)
- in order to build and install directly