Copy Pattern Template (see bellow).
Replace NAME, all the ET_Builder_Pattern_NAME in the template (3 places).
Replace TITLE in the template (2 places).
Replace PRIORITY in the template, lower number will make it show-up early in Pattern Style Dropdown list in the VB.
Save in a new file, e.g: some-name.php, in this folder, add/commit to the repository.

Tip:

For NAME, if it's multiple words like Diagonal Lines, use _ to join, e.g Diagonal_Lines.
For filename, if it's multiple words like Diagonal Lines, use - to join and make it lower case, e.g diagonal-lines.php.
Once new filename.php placed in this folder, the new pattern would automatically appear in the VB (just refresh).
default', 'default-inverted', 'thumbnail' should only contain all tags inside the <svg></svg> file, e.g:

'thumbnail'            => '<path d="M28,28H56V56H28ZM0,0H28V28H0Z"/>',

Pattern Template:

<?php
/**
 * Background Pattern Style - TITLE.
 *
 * @package Divi
 * @sub-package Builder
 * @since ??
 */

if ( ! defined( 'ABSPATH' ) ) {
	die( 'Direct access forbidden.' );
}

/**
 * Class ET_Builder_Pattern_NAME
 *
 * @since ??
 */
class ET_Builder_Pattern_NAME extends ET_Builder_Background_Pattern_Style_Base {
	/**
	 * Configuration.
	 *
	 * @return array
	 */
	public function settings() {
		return array(
			'label'      => esc_html__( 'TITLE', 'et-builder' ),
			'svgContent' => array(
				'default'          => '',
				'default-inverted' => '',
				'rotated'          => '',
				'rotated-inverted' => '',
				'thumbnail'        => '',
			),
			'width'      => '11px',
			'height'     => '11px',
			// Replace following PRIORITY with number (1-9) and uncomment to make it on top 9 list.
			// phpcs:ignore Squiz.PHP.CommentedOutCode.Found -- temporary comment.
			// 'priority'   => PRIORITY,
		);
	}
}

return new ET_Builder_Pattern_NAME();

Last Updated: Mar 10, 2022.