The specific tasks involved in viva prep – reading your thesis, making notes, rehearsing and so on – don’t need to happen until after you’ve submitted your thesis. Until then your goal has to be getting your thesis written and finished.
Viva prep is not purely mechanical though. Being ready isn’t simply completing a list of activities. Being ready is also a matter of confidence. Do you feel capable? Do you feel like you can engage with your examiners’ questions?
You don’t need to prepare for the viva until after you have submitted but it’s a valuable investment to do things to build your confidence over a long period of time. The more that you reflect on your success and remind yourself of it, the more you will feel ready for your viva when it arrives.
Reflect on your success. Remind yourself of what you have done. Do things that help you to feel confident.