Doing a PhD and writing a thesis is like finding your way to the centre of a maze. You might have some ideas about how to get there when you start, but it will still take work to make it through. You can go down wrong paths, get lost, but with time and effort you’ll get there.
Preparing for the viva is finding your way back out of the maze. It takes less time, but you have to check your way as you go. Just because you made it in, it doesn’t mean you know the shortest way back. You can still get lost. But you have a lot of experience to draw on now.
Describing the maze is the viva. How it looks. How you got there. Why you decided to walk it in the first place.
Walking back out of the maze will help you make sense of how you got in. Checking back over the twists and turns will make explaining the route to someone else a much easier task.