Viva preparation is not the hardest part of the PhD journey by a long way, but it requires a little thought for it not to be overwhelming.
What do you need to do? Where do you see the gaps for yourself? If you have trouble remembering things, then re-read them and make useful notes. Consider what you could add in annotations to help your thesis more useful. Think about when to rehearse with a mock viva or a good chat with friends. Make a list of what you really need to do, rather than work in an ad hoc way.
When do you start? Consider your responsibilities, consider when you could fit in thirty minutes to an hour a day. It’s better to view prep as a daily practice in most cases, a gentle lead up to feeling you are prepared, rather than a last minute cram to get “everything” done.
It could feel like there’s lots to do to be prepared for the viva. In comparison to the time and effort of the rest of your PhD there’s really very little. If you have a busy life already then it’s worth planning in advance how you will make space for it.
But remember: you can do it.