A helpful part of viva preparation is making your thesis ready for the viva. Then, throughout your discussions with your examiners you have a silent partner, able to support how you think and what you say.
You can write in the margins, stick Post-it Notes in, highlight and underline and do anything to annotate the book and make it better.
How can you make your thesis easier to navigate?
How can you make things easier to find?
How can you make things easier to see at a glance?
Those are the key questions that help you make a well-annotated thesis. You have to analyse what you have and annotate to make it better for you in the viva. Easier navigation could be achieved with Post-it Notes or highlighter tabs. A brief note at the start of each chapter could make things easier to find. Consistent highlighting practice might help you find key words and ideas at a glance.
Analyse and annotate. Find a system that works for you. Make a list, do the work and then benefit.