Adding an additional device to my iCloud account

Ideally your wife should have her own ID, you can invite her to family sharing from your Account if you wish which will let you and her share the purchases between you, but if you try to share an ID you may well get in a bit of a mess with contacts, calendars and other personal info.

WC's post is the recommended answer, but to answer your question, on the iPad, Settings/iCloud and sign in with the desired Apple ID. If there is already one signed in, sign out/in.