Then define the event in the form eg.
Event ButtonClicked
and call it when the button is clicked:
RaiseEevent ButtonClicked

You need to make sure you dim the form in the class with WithEvents, then you should see the form in the object-dropdown box (left dropdown box), and the event in the right dropdownbox inthe code editor. Basically it works the same as with classes (since a form is a class).
Oh I assume you know that when you dim the form withevents, you need to Set fFormVar = New MyForm and also call that fFormVar.Show