In order to use afterstep.. create a directory called 
~/GNUstep/Library/AfterStep and issue the command:

cp -R /usr/X11R6/share/afterstep/* ~/GNUstep/Library/AfterStep

Replacing /usr/X11R6 with the location of your X tree or installation
base dir (if different).

Then place afterstep in your .xinitrc or .xsession file, replacing your
current window manager.

More information about using afterstep is available at:

http://www.afterstep.org/FAQ/