NI - Saving information via Forms

I am trying to learn how to save information to the database by using a form. My goal is to be able to modify the registration form and add more fields. I know there is a plugin that does this but I feel I will more control if I learn how to code it on my own. 

Since there are quite the files involve in order to achieve this, please be specific about the files that need to be modified and their paths. 

How much do you guys charge to teach how to do something like this?

 

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