Gnome15 requires a number of open source platforms and libraries. You will need all of these to compile or run Gnome15 from source. Your distribution will most likely have packages for a lot of these, so check there first. Check the pre-requisites before proceeding.
There are 2 ways you can get the source to Gnome15. Firstly, you can download the latest released source tarball from the list of files below. Alternatively, you can retrieve it from the Launchpad source repository, using the bzr command (you will need to install the Bazaar Version Control System) :-
bzr branch lp:~tanktarta/gnome15/stable gnome15
Most of Gnome15 uses autotools. Run the commands below, or for a more detailed explanation you should read the Installation Manual.
Then, depending on your distribution, use either :-
sudo make install
.. or ..
su -c make install