It’s great if you have read your thesis, made notes, created summaries. You’ve probably got a beautiful mental model of your research. Can you use it though? How will you do in the viva?
Find opportunities to field test your knowledge and your skills. The mock viva is a great opportunity to do that, as are discussions over coffee and seminars for interested friends. Get confident answering questions by getting people to ask you questions.
Sounds simple; is simple.