Mapped Doctype (Copy data from one doctype and set into another)

Use Case :-

I have 2 doctypes called Job Applicant & Interview.

I create one button on Job Applicant doctype called Make Interview. Now what i want that when i click on this button then the some fields are copied from Job Applicant to Interview Doctype Like Mapping I tried (get_mapped_doc method) but i dnt have any example to understand how it will works?? Can anyone give some example for this? or how i’ll do this?