You can’t be ready for your viva until you’ve at least submitted your thesis.
You need to have spent some time doing various viva prep tasks.
To be ready you have to feel some amount of confidence for the viva.
To be ready you have to have some idea of what to expect.
Ready isn’t an irreversible state: you have to maintain it.
Once you do feel ready, consider: what could you do to hold that feeling? What can you do to keep feeling confident?