Creating Conditional Activation page

I generated userRegistration, userlogin, userlogout, userEmailPW, userProfile pages using SA. Also, I have an activation page with one form field element(Activation Number). Below is what I will like to accomplish:

1) When my first time registrant completes the userRegistration form, the registrant is redirected to Activation page(that I can do).

2) An email is immediately send to the form owner indicating that a first time registrant has registered.

NOTE: The registrant cannot login yet until the registrant is provided an Activation number. If the first time registrant tries to login, the registrant will be redirected to the Activation page until he completes the activation page. That is, new or first time registrant should follow the schematic below

Registration page --------->Activation page --------->login page

3) After receiving the email, the form owner sends the Activation Number to the client. The form owner will manually enter the Activation number in the dbase and sends the same number through email to the registrant. When the registrant enters the provided activation number in the activation page, then the registrant is the redirected to the login page. A conditional statement is set such that the First time registrant MUST follow the above schematic, otherwise the registrant will not be able to login.

4) Thereafter, the registrant can login. I know I have to use session variable in the Activation page to verify the provided Activation number, but what should I do such that new registrant follow the schematic?
But if a client had previously registered, then the client can login directly without necessarily going through the above schematic.

Any help will be greatly appreciated
Thanks in advance