// create the form fieldset
$form = Fieldset::forge('form');
// load the properties from the model
$form->add_model('Model_Registration');
// make sure the custom rules can be found
$form->validation()->add_callable('Model_Registration');
// run the validation
$form->validation()->run();
This allows you to define basic validation rules, and usually that is enough.
Alternatively, if you need more complex stuff, you can also forge a validation object in the model, so you can add whatever you want to it. Like so: http://scrp.at/bwj. You can also use this when you don't need form objects. It looks like you're new here. If you want to get involved, click one of these buttons!