Well, I have used this in Slackware 11 Xorg 6.9. If you use Xorg 6.8.1 already have support for transparent windows and shadows.
The first is to edit the X11 configuration file in the folder /etc/X11 called xorg.conf
Add the following lines anywhere in the xorg.conf to enable transparency and shadows.
Option "Composite" "Enable"
Then save the file and restart X.
Now we must use some commands in a terminal.
The first command is:
xcompmgr -cf &
With this command will enable the effects of transparency and shadow, and the symbol & is to put the execution of the command in the background for further work on the same console.
With this command, the mouse pointer will change to the window to select a victim. With 5 you say that applies to the window opacity of 50%. Change to your liking.