Follow the installation instructions on the GitHub wiki for Sonarr. You need to install mono, then add the Sonarr keys/repos to the apt database, and install it:
sudo apt-get install libmono-cil-dev sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 0xFDA5DFFC sudo echo "deb http://apt.sonarr.tv/ master main" | sudo tee /etc/apt/sources.list.d/sonarr.list sudo apt-get update sudo apt-get install nzbdrone
Ubuntu 18 now uses systemd so follow the instructions here for autostart.
I created a nzbdrone user and added it to the Plex group (since I use Plex to stream everything).
sudo useradd -m -d /var/lib/nzbdrone --gid plex nzbdrone
The sonarr.service file that I created in /etc/systemd/system/ (for autostart, above) was edited to include username and the ‘plex’ group under the [Service] section.