The process and the goal.
Getting ready means reading your thesis, asking questions, checking things, making notes and building confidence. Being ready means feeling certain that your viva will be successful.
Before you sit down to get ready, think ahead to what you want to feel when you are ready. What does being ready mean for you? What would you have done? What would convince you that you’re ready for your viva?
When you have a sense of what it would mean for you to be ready for your viva, you can plan and do your preparations much more easily.