How can I create a custom Gutenberg block for my WordPress site?

<!-- BEGIN COPY / PASTE -->
    // JavaScript: Create a new block in a file like block.js
    wp.blocks.registerBlockType('myplugin/custom-block', {
      title: 'Custom Block',
      icon: 'smiley',
      category: 'common',
      edit: function() {
        return wp.element.createElement('p', null, 'Hello, World!');
      },
      save: function() {
        return wp.element.createElement('p', null, 'Hello, World!');
      },
    });

    // PHP: Register the block in your plugin or theme's functions.php
    function myplugin_register_custom_block() {
      wp_register_script(
        'myplugin-custom-block',
        plugins_url('block.js', __FILE__),
        array('wp-blocks', 'wp-element')
      );

      register_block_type('myplugin/custom-block', array(
        'editor_script' => 'myplugin-custom-block',
      ));
    }
    add_action('init', 'myplugin_register_custom_block');
    <!-- END COPY / PASTE -->