Install Mono

This will install the Mono package from the (L)Ubuntu repository.

Install

Installing Mono is a simple task. Just use the following command:
sudo apt-get install mono-complete libgdiplus

If you want to try C# development as well just add monodevelop, too.
sudo apt-get install mono-complete libgdiplus monodevelop

The installation will take some minutes.

Uninstall

Uninstalling is as simple as installing:
sudo apt-get autoremove monodevelope (only if you installed it)
sudo apt-get autoremove libgdiplus

Hint
libgdiplus removes mono-complete as well.