Nokia N9 is probably easiest phone to root in the world that is available to buy. All you need to do is go to Settings > Security > Developer Mod > Click On and confirm it. Simple as that :D
If you want to allow installation from non-store services go to Settings > Applications > Installation > And simply allow it.
Now just click to install a repository, you don’t need to do anything more.
As mention in the video N9 is a consumer product so this is turned off by default but it’s there if you ever wanted.