Use add_shortcode function to register shortcode for later use. The first parameter is a name of your shortcode, the second is a callback function where you can add the content to return.
functionwpshort_assemble_shortcode(){$message='Hello from the shortcode!';return$message;}/*
* Register shortcode
*/add_shortcode('my_shortcode','wpshort_assemble_shortcode');/*
* Use [my_shortcode] handle in a WYSIWYG editor to display it.
* Alternatively, you can use do_shortcode() function directly in code.
*/