How to declare and define a c++ function in separate header and cpp file

I have used the Dev-Cpp compiler to show how to declare a function in a header file, then define it inside a cpp file, and use the function in a third cpp file. Video on how to add two or more files to a C++ project Main article that references this video is here - http://hoven.in/Home/Videos/CPP-Video-Tutorials/header-files-and-linkage.html