bounce.cgi is, perhaps, the trickiest part of installation. The process (email aliases forwarding email to a CGI script) can vary greatly across hosts and may not be supported.
The first thing you should do is ask your web host how it's done on your server. Tell them you want all email to bounce@yoursite.com forwarded to a CGI script (bounce.cgi) in your CGI bin. They will let you know whether it's supported and, if so, let you know how to go about setting these up.
With email signup, you need to configure another CGI script for each list or sets of lists, entering this data in the "$list" variable in the script. After that, you can point one or more email addresses to that CGI script to facilitate signup with a blank email. You do not need to enter anything into the ListMail control panel when setting up an email signup script/address. The list being subscribed to simply has to exist.
Email signup is not required. Depending on how easy it is to set up the email aliases/forwarders required on your host, you may or may not want to use the feature. Form subscription has the added benefit of directing subscribers to a completely customizable thank-you page, keeping their attention on your web site.