On Celebrity's website log into your CP, select your cruise, click on "plan my cruise", scroll down to "dining" and click on "view all dining". Scroll down to "Complimentary Dining", click on "Celebrity Select Dining" and click on reserve. Like with RC you will need to complete the reservation process for each evening before selecting additional evenings.
On the app tap on the "person icon", tap "manage booking", fill in the reservation info, tap "find reservation" and scroll down to "main dining seating time", tap on that and select your dining preference and time.