The applet can be installed using the following steps:
sudo apt install libxkbcommon-dev just
git clone https://github.com/rwxroot/cosmic-ext-applet-sysinfo.git
cd cosmic-ext-applet-sysinfo
just build
sudo just installlibxkbcommon-dev is required by smithay-client-toolkit
On Arch Linux, the applet can be installed using the PKGBUILD cosmic-ext-applet-sysinfo-git, available on the AUR.