Prior to start, make sure that the server has been licensed to use fantastico.
ssh to your server and log in as root and do the following commands:
# cd /usr/local/cpanel/whostmgr/docroot/cgi
# wget http://www.netenberg.com/files/free/fantastico_whm_admin.tgz
# tar -xzpf fantastico_whm_admin.tgz
# rm -rf fantastico_whm_admin.tgz
Login to your WHM (if already logged in, just refresh the page)
From the menu, under Plugins, click Fantastico De Luxe WHM Admin (located at the bottom).
the rest should be self explanatory 