Set up one folder and call it what every you want but you will need that path and name in your form and formmail.php. Then have another seperate folder for your templates. It would be like www.yoursite.com/forms/formtemplates Put the forms and formmail.php and if you set it up fmbadhandler.php in the "forms" folder. Now your custome thank you or what ever page goes in "forms/fmtemplates" then you have to set your form up for that file.
Here are some settings in one of my forms to give you an idea. Formmail.php has the location of the templates so all it needs is the template name.
The following setting is in formmail.php
<input type="hidden" name="bad_url" value="fmbadhandler.php" />
<input type="hidden" name="this_form" value="feedback.php" />
<input type="hidden" name="good_template" value="feedback.tpl" />
<input type="hidden" name="bad_template" value="feedback_err.tpl" />
<input type="hidden" name="mail_options"
$TEMPLATEDIR = "fmtemplates"; // directory for template files; empty string
// if you don't have any templates