Just download the disk image and follow the instructions in the Read Me. The md5 checksum is 7367c911425d888b81cb66549bf62eb2. I have tested this distribution on Tiger, but I believe it should work on OS X 10.3.9. This launcher does not replace the default system WebKit, and you can run "standard" Safari at any time -- even at the same time -- by launching Safari the usual way.
The launcher is a simple perl script, adapted from the run-safari script that's included with the WebKit open-source package. It tells the system to use the local, new copy of WebKit when it runs Safari. The popular plug-in PithHelmet does not work with this application wrapper, but you can use the standard Safari with PithHelmet just fine.

