OVOS-Installer is the newest way to install OVOS and is probably the easiest way to install on an existing system.
You do need a few packages installed on your host system before starting.
sudo and sudo access for your user
After installing those packages, simply run a command and follow the on screen instructions.
sh -c "curl -s https://raw.githubusercontent.com/OpenVoiceOS/ovos-installer/main/installer.sh -o installer.sh && chmod +x installer.sh && sudo ./installer.sh && rm installer.sh"
More information on the OVOS Installer can be found on the README page in its Github repository.