Step one:
--------

Install packages like a C compiler, glib-2.0, flex and bison
TODO: complete this list

Step two:
---------

Vala is self-hosting so it needs another vala compiler to compile
itself.  If you don't have an existing vala installed (i.e. because
you're bootstrapping or cross-compiling), you can use the
vala-bootstrap module:

http://git.gnome.org/browse/vala-bootstrap

See also:

http://live.gnome.org/Vala/Release

Downloading and compiling the bootstrap module:

git clone git://git.gnome.org/vala-bootstrap
cd vala-bootstrap
./autogen.sh --prefix=/opt/vala-bootstrap --disable-build-from-vala --disable-vapigen
make && sudo make install

Step three:
----------

Compiling the newest Vala from the repository:

git clone git://git.gnome.org/vala
cd vala
export VALAC=/opt/vala-bootstrap/bin/vala
./autogen.sh --prefix=/opt/vala
make && sudo make install

Maybe you want to compile the new Vala with itself. Then you simply go over
it again:

git clean -dfx
export VALAC=/opt/vala/bin/vala
./autogen.sh --prefix=/opt/vala
make && sudo make install