In order to install it on RedHat, get it here. Then click on your RedHat Menu / System Settings / Login Screen. Click on the "Graphical Greeting" tab and click on "Install New Theme" (make sure you select it after it shows up in the list). That is it.
PS: If you wonder how to take a screenshot of the GDM (login) screen, this is the way it works for me:
echo "chvt 7 ; sleep 5 ; XAUTHORITY=/var/gdm/:0.Xauth DISPLAY=:0.0 import -window root /tmp/gdm-shot.png" > /tmp/capture
(This assumes your login screen is in VT7 CTRL+ALT+F7)
Now log off and at the GDM screen hit CTRL+ALT+F1. Log in as root and type sh /tmp/capture. The screen will switch to GDM and after a few seconds you will hear a BEEP. Log in and you will have the screenshot at /tmp/gdm-shot.png
Note: import is part of the ImageMagick package
Also, take a look at my GDM Theme HOWTO