If you know how to install Windows on Mac, basically you then know how to do it for Ubuntu.
Preinstallation: you have Bootcamp which allows you to partition the Mac disk, and you have an Intel Mac!
Installation: some features of Ubuntu are not supported on some Mac, Check the list to determine if you really need to install it on your Mac.Instructions on how to install Ubuntu are also available in the same location.
Postinstallation: a painful process to follow coz you have to deal with numerous things in order to have your Ubuntu work as you expected... This is one of the disadvantages (and joys) to use open source products -> you are the master.So take your own responsibility... Follow this link again to tune your system as you like.
Last note, now your Mac is dual-booted, I don't like the software used for booting, so I have to press ALT/OPTION to select the system I want to use -> Ubuntu is not called Ubuntu, but Windows in the list~~~