Must the missed 'Isha' prayer be made up, and at what time should it be made up? And is ghusl required to perform the subsequent prayers, or is wudu' sufficient?

You have committed a grave sin by missing prayers at their appointed times, for abandoning even one prayer until its time expires is a greater sin than adultery, theft, consuming alcohol, and murder. You must offer sincere repentance (Tawbah (definition) Nasuh) by regretting your actions, seeking forgiveness, and resolving never to return to such a sin. You are also obliged to make up all the prayers you have missed, and it is permissible to make them up at any time.