urgent help regarding to customising registration form

I need an urgent help regarding to customising registration form. Apparently there is no easy way for doing that. I started similar thread two days ago and I got one reply from Alexander Rohmann . I’ve been told to :

https://premium.wpmudev.org/forums/topic/there-would-you-please-advise-me-how-can-i-customise?replies=3#post-428151

1. You can directly change “registration.form.php” and add the new fields.

2. Membership registration forms still call the “signup_extra_fields” hook. So you can use this hook to add more content to the form without modifying Membership directly.

If you’re interested in doing this in code let me know and I can help you get started.

Then I asked for future help but haven’t received any reply yet. I wonder any other supports can handle this thread and give me required hint such as where to begin, what part of database I need to modify? how does registration form work? and how can I use the signup_extra_fields hooks etc…