Start with a fresh Ubuntu Server 16.04 installation.
NOTE: In the commands and files below, be sure to replace <yourusername> with your username and <yourgroupname> with your group name (probably the same as your username). Also, replace <yourhostname> with your server's hostname.
- Add the latest mono apt source (from instructions at http://www.mono-project.com/download/#download-lin-ubuntu):
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
echo "deb http://download.mono-project.com/repo/ubuntu xenial main" | sudo tee /etc/apt/sources.list.d/mono-official.list
sudo apt update