How can i implement a simple user define function in this

Question

 How can i implement a simple user define function in this program?

 

#include <stdio.h>
#include <string.h>

struct Books {
   char  Name[50];
   char  author[50];
   char  subject[100];
   int   book_id;
};

int main( ) {

   struct Books Book1;        /* Declare Book1 of type Book */
   struct Books Book2;        /* Declare Book2 of type Book */

   /* book 1 specification */
   strcpy( Book1.Name, "C Programming Learning ");
   strcpy( Book1.author, "Nuha Ali");
   strcpy( Book1.subject, "C Program");
   Book1.book_id = 6495407;

   /* book 2 specification */
   strcpy( Book2.Name, "Telecom  Learning");
   strcpy( Book2.author, "Zara Ali");
   strcpy( Book2.subject, "Telecom");
   Book2.book_id = 6495700;

   /* print Book1 info */
   printf( "Book 1 Name : %s\n", Book1.Name);
   printf( "Book 1 author : %s\n", Book1.author);
   printf( "Book 1 subject : %s\n", Book1.subject);
   printf( "Book 1 book_id : %d\n", Book1.book_id);

   /* print Book2 info */
   printf( "Book 2 Name : %s\n", Book2.Name);
   printf( "Book 2 author : %s\n", Book2.author);
   printf( "Book 2 subject : %s\n", Book2.subject);
   printf( "Book 2 book_id : %d\n", Book2.book_id);

   return 0;
}