Installing QGIS with GRASS plugin on Ubuntu?

I already tried to install QGIS 2.8.3 using the procedure described on: http://www.qgis.org/en/site/forusers/alldownloads.html#debian-ubunt, but grass-plugin do not compare on plugin list!

I tried both repositories: ubuntugis or debian, but nothing! I also cleaned the previous QGIS 2.6 installation (remove old ppa) on my UBUNTU 14.04.

Does anyone tell me a complete and verified fresh procedure (step by step) to do a good install that allows grass-plugin (grass 6 or 7 it's the same)?

19 ago update : I've done exactly this steps (before I cleaned previos install and repositories):
sudo apt-get install python-software-properties
sudo add-apt-repository 'deb http://qgis.org/debian trusty main'
gpg --keyserver keyserver.ubuntu.com --recv DD45F6C3
gpg --export --armor DD45F6C3 | sudo apt-key add -

sudo apt-get update
sudo apt-get install qgis python-qgis qgis-plugin-grass grass-gui grass-doc

So, I correctly installed : QGIS 2.10.1, GRASS 6.4.2 and gras-plugin working !! But ... mysteriously in grass-toolbar: "add raster layer" and "add vector layer" do not compare !! ....I d'nt understand. What's the matter ?

Using repository 'http://qgis.org/debian trusty main', it is installed the Qgis 2.10 release ... if I am not mistaken !

If it's possible instead, I want to install Qgis 2.8.3 LTR with grass-plugin working (GRASS 6 or 7 it'snt important).