How to create a MU Register Steps page?

I want to make a register page for wordpress MU.
here is workflow:
step1. create account;

step2. congratulation page with two buttons:create a site & back home;

step3.site creation page;

step4.success!

Now,I've copied wp-signup.php in my theme folder ,made it as a page-template(I can custom signup url).then,301 redict original wp-signup.php to new page what I created by page function.

the problem is?how can I insert STEP.2 page in register flow. it skipped to step3 when clicked the signup button in step1 page.

Have some solution?