Which package to choose
Take the .deb for Ubuntu, Debian, and Linux Mint, and the .rpm for Fedora and other RPM-based distributions. If you are not sure, download the AppImage: a single file that runs on almost any distribution without installation. Raspberry Pi 4/5 and other ARM devices have a separate ARM build.
How to launch it and sign in
Open https://jimm.online/qip, pick Linux, download the package, and follow the guide on the page. After launch, sign in with your UIN and password; the client connects to the icq.jimm.online server. If you do not have a UIN yet, register on https://jimm.online.