Very simple. I will write for you a simple example.
main.c
#include <stdio.h>
#include "simple.h"
int main (void) {
printToken("hello everyone, i hope this help you");
return (0);
}
simple.h
#pragma ONCE
// the header usually contains only prototypes or global variables
void printToken(char *string);
simple.c
#include <stdio.h>
#include "simple.h"
// this file contains the functions which "simple.h" reads from.
void printToken(char *string) {
printf("%s", string);
return;
}