Originally from the ROS wiki.
ROS is an open-source, meta-operating system for your robot. It provides the services you would expect from an operating system, including hardware abstraction, low-level device control, implementation of commonly-used functionality, message-passing between processes, and package management.
Get dependencies, and create alternate python dir to workaround shebang line in scripts that points to python3. Thanks to DavidS. Symlink libboost_filesystem.so.1.47.0 to libboost_filesystem-mt.so, not an ideal solution, but it works. Compiling gcc45 may be unneccesary-remove from here if it is.
pacman -S python2 cmake wget python-yaml python2-distribute python-yaml pil python-numpy yaourt -S gcc45 log4cxx mkdir /usr/local/bin/alt ln -s /usr/bin/python2 /usr/local/bin/alt/python ln -s /usr/lib/libboost_filesystem.so.1.47.0 /usr/lib/libboost_filesystem-mt.so
Follow the ROS wiki afterwards.