A beta release is available here: Graphem 0.3.2 (Source code)
You can also get a current (and probably broken) revision of the source via WebSVN or anonymous checkout:
svn checkout svn://svn.berlios.de/graphem/trunk
To compile Graphem, you need Qt4 (version 4.3 or newer) and QCA 2 development libraries, which are included in many Linux distributions. On Debian/Ubuntu you need to install libqt4-dev and libqca2-dev. After installation, you can compile with:
qmake-qt4 make
Once compiled, start Graphem by typing ./graphem and click on New Pattern... in the File menu. You will see something like this:
For now, click Use Normal Mode. You can now start drawing your key pattern, which could look like this:
Here, I started by drawing an "L", releasing the mouse button, and drawing a mirrored "L" on top of the first. Red arrows indicate that you were just moving the mouse somewhere else, while white arrows are actual strokes. You should limit yourself to the eight possible directions for strokes (horizontal, vertical and diagonal) while drawing your key pattern, as recognition will become painful if you don't. ;-)
Especially on corners Graphem likes to sneak in small arrows which don't really belong there. You can remove them by clicking on Delete Last Stroke. Generally you should strive to make your key pattern as "clean" as possible and include only those elements you actually want to draw each time you login.
When you're done, click OK and use the box on the right side to check wether Graphem recognizes your key. If you're convinced it's working correctly (i.e. the recognition rate is high), you can save (File -> Save) and test the screen locker by executing ./graphem --lock.
To get a list of other command line options, you can use the --help argument. You also might want to check the README.
For more information, please see the Project Page.