Using Interfaces in Kohana 3.1.3

I'm trying to build a form wizard in Kohana and am learning a bit as I go. One of the things that I've learn might work best is utilizing a state pattern in my class structure to manage the different steps a user can be in during the form process.

After doing some research, I've been thinking that the best approach may be to use an interface and have all of the steps act as states that implement the interface. After a state validates, it will change a session variable to the next step, which can be read upon the initial load of the interface and call the correct state to use.

Does this approach make sense? If so, how the heck do I make it happen (how do I best structure the filesystem?)

Here is the rough start I've been working on:

<?php defined('SYSPATH') or die('No direct script access.');

/**
* Project_Builder @state
* Step_One @state
* Step_Two @state
**/

interface Project_Builder 
{
    public function do_this_first();
    public function validate();
    public function do_this_after();
}

class Step_One implements Project_Builder {

    public function __construct
    {
        parent::__construct();

        // Do validation and set a partial variable if valid

    }

    public function do_this_first() 
    {
        echo 'First thing done';
        // This should be used to set the session step variable, validate and add project data, and return the new view body.
            $session->set('step', '2');


    }
    public function do_this_after()
    {
        throw new LogicException('Have to do the other thing first!');
    }

}

class Step_Two implements Project_Builder {
    public function do_this_first()
    {
        throw new LogicException('Already did this first!');
    }
    public function do_this_after()
    {
        echo 'Did this after the first!';
        return $this;
    }
}

class Project implements Project_Builder {
    protected $state;
    protected $user_step;
    protected $project_