I had no idea it was this simple. Basically add a function to functions.php (in a theme folder, like wp-content/themes/yourtheme/functions.php) and add the add_shortcode line as below.
Once it’s been added there, you can use it with [your_shortcode].
For instance, here’s the code I use for generating a random number for the “hit counter” on my main page:
// shortcode to generate a random number
function random_number() {
// generate a random number
$number = rand(1000000,10000000);
// add commas in the right places
$number = number_format($number);
return $number;
}
add_shortcode('random_number', 'random_number');
// now you cause use it with 7,812,741