It’s one thing to know all of your research really well, and to know generally what happens in the viva (expectations, structure and so on). You can have a good picture of it all in theory – but then stumble when you come to the reality of being asked questions and being forced to think, ponder, respond and engage.
Mock vivas, meetings, seminars, presentations, conversations, impromptu Zoom-meetups, mini-vivas – take and find every opportunity you can to share your research and be asked questions about it.
Theory will only take you so far. Build confidence for the reality of the viva.