Debian source packages can't actually be "installed", they are just unpacked in whatever directory you want to build the binary packages they produce.

Source packages are distributed on most of the same mirrors where you can obtain the binary packages. If you set up your APT's sources.list(5) to include the appropriate "deb-src" lines, you'll be able to easily download any source packages by running

apt-get source foo

To help you in actually building the source package, Debian source package provide the so-called build-dependencies mechanism. This means that the source package maintainer keeps a list of other packages that are required to build their package. To see how this is useful, run

apt-get build-dep foo

before building the source.

You will need all of foo_*.dsc, foo_*.tar.gz and foo_*.diff.gz to compile the source (note: there is no .diff.gz for some packages that are native to Debian).

Once you have them (How do I install a source package?, Section 6.13), if you have the dpkg-dev package installed, the following command:

dpkg-source -x foo_version-revision.dsc

will extract the package into a directory called foo-version.

If you want just to compile the package, you may cd into foo-version directory and issue the command

dpkg-buildpackage -rfakeroot -b

to build the package (note that this also requires the fakeroot package), and then

dpkg -i ../foo_version-revision_arch.deb

to install the newly-built package(s).