To be more specific, provided you do the work it’s hard to not be prepared. Viva prep doesn’t need to take a lot of time, maybe twenty or thirty hours in total, and the type of work that helps is really clear.
Read your thesis, make some notes, check some papers, rehearse for the viva.
Do the work and you’ll be ready.
The only sense of “wrong” prep comes from the environment you might make. There’s no law that says when you have to start getting ready for the viva. You could start your prep a few days before – but that might be a very pressured situation! You could do an hour a day for the two months leading up – but that might spread things out a little too much, or mean that you invest more time than you need.
It’s hard to get prep wrong, but you have to think about the way you do the work to get it right for you.