If you are just starting to setup your box, its recommended that you download a fresh Mythbuntu image and start from scratch.

However, one of the best features of Mythbuntu is that all development happens directly in the Ubuntu archives. This means that you can add all Mythbuntu roles to an existing Ubuntu box.

Prerequisites

1) Universe and Multiverse must be activated.

Information on how to setup your repositories is available wiki

2) You must be running Ubuntu 7.10 (Gutsy) or later.

You can check your version by Choosing System, Followed by "About Ubuntu"

Installing

Click this link to install Mythbuntu:

Configuring

Choose System, Administration, Mythbuntu Control Centre to open up the Control Centre. From here you can modify all options that are already setup during a normal Mythbuntu Installation.