Schedule Your Prep
You might not know your viva date at submission. Confirmation could come weeks after you have submitted. In some cases a candidate might only get two weeks warning of a viva date (assuming that the date works for them too).
Sketch out a plan for viva prep at submission. This might have some blanks. It might not have precise dates. But consider how busy you are and what commitments you have already.
When, where and how would you fit in viva preparation?
A schedule could have tasks broken down date by date or be a list of points you have to tackle. Any helpful structure you can give yourself at submission will help you appreciate the scope of what needs to be done, what questions you need to ask yourself and what further planning you need to do.
Then you need to do the work!