functions.php

class WC_Custom_My_Account_Tabs extends WC_Query {
	/**
	 * Adds main filters and actions and inits the endpoints.
	 */
	public function __construct() {
		add_action( 'init', array( $this, 'add_endpoints' ) );

		if ( ! is_admin() ) {
			add_filter( 'query_vars', array( $this, 'add_query_vars' ), 0 );
			add_filter( 'woocommerce_account_menu_items', array( $this, 'edit_navigation' ) );
			add_action( 'woocommerce_account_custom_endpoint', array( $this, 'add_custom_tab_content' ) );
		}

		$this->init_query_vars();
	}

	/**
	 * Inits the query vars for WooCommerce
	 */
	public function init_query_vars() {
		$this->query_vars = array(
			'custom' => 'custom',
		);
	}

	/**
	 * Edits the navigation in the page My Account, removing, editing and adding new tabs.
	 *
	 * @param  array          $items
	 * @return array
	 */
	public function edit_navigation( $items ) {
		// Remove tabs
		unset( $items['downloads'] );

		// Rename tabs
		$items['edit-address'] = 'My Custom Title';

		// Add tabs
		$items['custom'] = 'Custom Tab';

		return $items;
	}

	/**
	 * Prints the custom tab content from a template in theme-name/woocommerce/myaccount/
	 */
	public function add_custom_tab_content() {
		wc_get_template( 'myaccount/custom.php', array(), '', get_stylesheet() . 'woocommerce/' );
	}
}

new WC_Custom_My_Account_Tabs();