This little How-To is intended for Snow Leopard and ShairPort 0.03.
All credits to James Laird for his reverse engineering work. My best regards to him and his girlfriend.
Download ShairPort 0.03.
Download and install MacPorts.
Download and install Apple Developers tools (XCode): register to the iOS or Mac development program and grab it for free or purchase it from the Mac App Store.
Open Terminal (you can find it in Utilities subfolder in Applications folder) and write
sudo port install openssl libao nss avahi
then write your account password (this will take a lot of time!)
When the command finishes write
sudo cpan HTTP::Request
and write your account password (write yes two times if it asks for auto configuration and best mirror).
sudo cpan HTTP::Message
sudo cpan Crypt::OpenSSL::RSA
sudo cpan IO::Socket::INET6
Now we need to launch some processes, so in terminal write again
sudo launchctl load -w /Library/LaunchDaemons/org.freedesktop.dbus-system.plist
sudo launchctl load -w /Library/LaunchDaemons/org.freedesktop.avahi-daemon.plist
sudo launchctl load -w /Library/LaunchAgents/org.freedesktop.dbus-session.plist
in the ShairPort-0.03 directory (use cd to navigate through folders in terminal) write
Edit the shairport.pl with and set a custom machine name on line 7 (this is what will appear on the iOS app or iTunes speakers list).
Each time you want to start ShairPort you need to open terminal, navigate to the shairport-0.03 folder and run
Surely someone will create a full featured application to easily start and stop ShairPlay, but for now it should be enough to listen you music on your mac(s).