After installing varnam, you will need to enable Varnam IBus Engine for easily start typing on any app system-wide.
Most GNOME systems are well-integrated with IBus by default.
If you’re not using GNOME, you will need to setup IBus. ArchLinux Wiki has good information on setting up IBus: https://wiki.archlinux.org/title/IBus
IBus setup might be a bit tricky in KDE. It can be easily setup in ArchLinux by installing ibus-input-support.
You can also choose a keyboard shortcut to easily switch input methods. The default key-combo is Meta Key (Windows Key) + Space
You can find tips and more information about Varnam IBus Engine here.
Install for Mac