Creating a form which filters out projects that don't have assignments

Creating a form which filters out projects that don't have assignments

Hello - 
I'm trying to create a report that will show me only projects for which I have no one assigned.
Assignments is a separate Form from my Projects Form. The Assignment Form references the project that it's for, so there must be a way to filter just projects that don't have any assignments, but I'm not sure how. Probably involves creating some kind of flag when an assignment is created.

Can someone help? and talk me through it? I'm a newbie at this, so I need baby talk... :)

thanks in advance.