Take Ubuntu or Debian for example:
(1) Get the building dependencies:
sudo apt-get build-dep alsa-driver
(2) Download alsa-driver-*.tar.bz2 from ALSA Project and extract.
(3) Replace files in alsa-kernel/pci/cs46xx/ by the patched files from the following zipped package (the patch was applied on top of ALSA 1.0.18a): cs46xx.zip.
(4) Build and install
./configure --with-cards=cs46xx --with-sequencer=yes ; make ; sudo make install
(5) Block the onboard sound card by putting in /etc/modprobe.d/blacklist.conf the following code:
# to use Blackgold II instead of on board sound card
(6) Reboot, and we're done!