You can read books about the viva. You can ask your supervisors for their advice. You can talk to friends and colleagues about their experiences. Get lots of recommendations.
And then you have to do something.
Take all the ideas and take action. Take everything you know about your research and thesis, factor in everyone’s advice, hints and helpful suggestions and take action.
Action that helps you remember what you need.
Action that helps you reflect on what you’ve done.
Action that helps get you one step close to where you need to be for talking to your viva.
At some point more knowledge and advice isn’t enough. You have to do something to help you get ready.