On the script install screen, if you click "info," it will link you to the application web site for additional tutorial and documentation.  The form mail script info page is located at:

http://www.scriptarchive.com/formmail.html

When installing a script, it will ask for the folder/directory you wish to
install to -- this must be a new/unique folder (i.e. not the cgi-bin folder).

The script needs to be installed per site, not system wide; however, the way many scripts work is such that you could create it on one domain
and link to it inside other web sites/domains (i.e., you could use 1 installation for everyone). View the link above for further tips on how to use the software/script.