I have read about this on XDA Developers, the articles were a few months old though.
Is there an official guide from Ubuntu on how to do this?
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The short answer is no, there is not. At the moment, the only release available is not for commercial or normal use, and for developers only. It's very unstable and shouldn't be done by those who are not serious about developing with that software and hardware.
Nexus 7 Installer
Because you do not have a Nexus 7, this isn't compatible, but you may find it interesting to learn more. This could also be an option for you, if you're willing to invest in buying a Nexus 7.
The Nexus 7 installer is not for commercial or normal use, and for developers only. It's very unstable and shouldn't be done by those who are not serious about developing with that software and hardware. Just like I said above, it's only for people really wishing to commit to this and people who won't cry about the countless bugs it has.
The Nexus 7 installer installs the ARM version of Ubuntu onto the device. That means it's the actual desktop OS, and isn't designed specifically for mobile devices.
If you wish to learn more about this you can use the sources below:
Installing Ubuntu manually on your device
The other option you have is installing the actual desktop OS onto your phone, which can be easy depending on what method you use. I have seen it in real life, so it does work. But doing this is completely your choice, and if you 'brick' your device or it breaks, you are not insured or protected. No-one will replace it or reimburse you, it's purely your own risk. Zedomax - a YouTuber does many tutorial videos on how to install Ubuntu like this on many different devices.
If you wish to do this, you can view the links below:
Only a couple days ago there was an announcement from Mark Shuttleworth about the new Ubuntu Phone OS! Since not much has been revealed, and no-one really knows a whole heap about it - I can gives you links and links and more links. It's a pretty interesting read and watch.