Look around you. There are lots of experts, able to share ideas, advice, experience. All you have to do is ask.
Ask specific questions about viva experiences to learn what they’re like.
Ask your supervisor and other academics how they approach being examiners.
Ask your graduate school about regulations and expectations.
Ask friends and colleagues for helpful questions to practise.
And ask yourself who the real expert on your thesis must be.
Who knows more about your research than anyone else?