The same way you add new users into the "users" table. It doesn't sound like you're very familiar with PHP and MySQL which means it's going to make this difficult for you. When you add new data to the "users" tab, it'll return the user id that was injected since "user_id" is an auto increment if I recall correctly. You use that user id and insert a new row into "user_data". If you don't understand how to do this, then I think you need to look into PHP and MySQL a bit more before attempting to finish your web registration page.