OK here's a step by step guide.
First of all you need to edit the FormMail.pl script to reflect your server's setting. The important bits of information are your path to sendmail (your host will tell you this) and the path to Perl (which needs to be in the #!/usr/bin/perl line at the top - again ask your host). You also need to add your domain name to the @referrers bit. Basically find the following lines and change them to reflect your settings:
#!/usr/bin/perl (the top line)
$mailprog = '/usr/lib/sendmail';
@referers = ('yourdomain.com');
That is ALL you need to do to the FormMail.pl file - you don't need to change anything else.
Upload it to your server's CGI bin and CHMOD it to 755 - if you don't know how to CHMOD stuff look up "CHMOD tutorial" on google.
That's the script set up. Now you need to creat an HTML page with the form that you would like to e-mail. Here's an example page:
There you go - that will display a form asking for user's comments which will be e-mailed to you when they click the button.
<form action="/cgi-bin/FormMail.pl" method="POST">
<input type=hidden name="recipient" value="email@example.com">
<input type=hidden name="subject" value="Your Subject">
Now you can put the fields you want in the form - here are some examples:<p>
Your Name: <input type=text name=realname><p>
Your E-Mail Address: <input type=text name=email><p>
Your Comments: <textarea name=comments rows=5 cols=30></textarea><p>
<input type=submit value="Submit Form">
There are lots of other "hidden" fields you can add to the script if you want that will change the way the script behaves. A full list of them is available here:
Hope that helps,