Most of the instructions are as described on the cern twiki
https://twiki.cern.ch/twiki/bin/view/Atlas/WorkBookInstallingAtlasSoftware
but some specifics:
export PATH=/usr/nevis/gcc-3.4.6/bin:${PATH}
pacman -pretend-platform SLC-4 (this needs to be done after creating the directory for the release, but before the pacman -get)
then
pacman -allow tar-overwrite trust-all-caches -get am-BNL:xx.xx.xx
-- GustaafBrooijmans - 04 Mar 2008