for your spouse, you reply back on mail from which you received the appoint details with letter. Give all the details as you gave while scheduling your appointment with a request to schedule your spouse's appointment. VFS will schedule and will share the appointment details on you mail itself.
Alternatively, take your spouse along with you at your scheduled slot, enrol her for PPR premier service by paying INR 3050 in cash and they whill give you the immediate slot for your spouse. (this is what I did)