In order to create your own header file, we create a function and save it with .h extension. This file contains only function and nothing else.
In the Main file which you have to save as .c extension and where you want that function to work. We will write a line #include"filename.h" use the function same as it is declared in the same file.
That's it! You have learnt how to create your own header file and how to use.