You have to do this all the time for your viva.
Take the first step to the viva when you submit.
Take the first step with your prep by sitting down and getting your thesis out again; or by asking for help (plenty of people can help but you have to ask for their support).
If you find a mistake – more than a typo – you have to take the first step, however tricky or uncomfortable, to figuring out what makes it right.
If you want a mock viva, you probably have to email and ask when your supervisor might be free.
If, in the viva, you are stumped by a question, you have to take the first step to responding to it. That could be awkward, you might feel pressured, but you have to do it.
All of which is a long way of saying, that for all the puzzles, problems and challenges you find with your viva, you have to take the first step to resolving them.
There’s no-one else to do it, but also no-one else who could do it.
So take the first step.
(and if you don’t feel you’ve found any puzzles, problems or challenges, take the first step towards finding some, because they’re there…)