How to Ordering by user meta

I am learning WordPress meta query. I am trying to fetch users with asc order of last_name first_name. My code is like below.

if($_POST['orga_id']) {
        $users_query = array(
            'meta_query' = array(
                'relation' = 'AND',
                array(
                    'relation' = 'OR',
                    array(
                        'key'     = 'first_name',
                        'value'   = $_POST['search'],
                        'compare' = 'LIKE'
                    ),
                    array(
                        'key'       = 'last_name',
                        'value'     = $_POST['search'],
                        'compare'   = 'LIKE',
                    ),
                ),
                array(
                    'key'       = 'organisation',
                    'value'     = $_POST['orga_id'],
                    'compare'   = 'LIKE',
                ),
                array(
                    'key'       = 'available',
                    'value'     =  1,
                    'compare'   = '=',
                ),          
            )
        );
    } else {
        $users_query = array(
            'meta_query' = array(
                'relation' = 'AND',
                array(
                    'relation' = 'OR',
                    array(
                        'key'     = 'first_name',
                        'value'   = $_POST['search'],
                        'compare' = 'LIKE'
                    ),
                    array(
                        'key'       = 'last_name',
                        'value'     = $_POST['search'],
                        'compare'   = 'LIKE',
                    ),
                ),
                array(
                    'key'       = 'available',
                    'value'     =  1,
                    'compare'   = '=',
                ),          
            )
        );
    }

How can I fetch users with asc order of last_name first_name ?