How can I create a custom Gutenberg block for my theme?

<!-- BEGIN COPY / PASTE -->
    // JavaScript: Registering a custom block
    wp.blocks.registerBlockType('mytheme/custom-block', {
      title: 'Custom Block',
      icon: 'smiley',
      category: 'common',
      edit: function(props) {
        return wp.element.createElement('p', {}, 'Hello, Gutenberg!');
      },
      save: function(props) {
        return wp.element.createElement('p', {}, 'Hello, Gutenberg!');
      }
    });

    // PHP: Enqueue block editor assets
    function mytheme_enqueue_block_editor_assets() {
      wp_enqueue_script(
        'mytheme-blocks',
        get_template_directory_uri() . '/blocks/custom-block.js',
        array('wp-blocks', 'wp-element')
      );
    }
    add_action('enqueue_block_editor_assets', 'mytheme_enqueue_block_editor_assets');
    <!-- END COPY / PASTE -->