Custom wp-signup.php with 3 page signup process ?

Hi all,

I have a requirement which goes like this for multisite users creates there blog

Basically its a 3 page signup process for logged in users to create there own blog

1.First page should have the site name, site title and site description and i need one next button

2.Onclick of next button it allows users to choose a theme,Then one more click button which takes to the next page

3.In the third page users need an option to customize the theme and then if they click submit the blog for the users should be created .

Can anyone please tell me how to customize the wp-signup.php or add a new template to make this happen.I am trying it from two days not getting how to do .Please can anyone help me out in this ,Thanks anyways for reading.