In most cases the following use is sufficient
$form->setModel('SomeModel');
If you want to import fields from multiple models, you can use this:
$ctl = $form->importFields($model,array('name','surname');
and if you want to use your own class based on this one, syntax is:
$ctl = $form->add('Controller_MVCForm_Derived')->importFields($model,array('name','surname'));
You can subsequently call importField() to add additional fields such as:
$form_field = $ctl->importField('age');
which will return newly added form field.