Make a little plan for the weeks leading up to your viva. It has to include reading your thesis, annotating it usefully, writing some summaries, reading important papers and rehearsing for talking with your examiners.
Do the work in a way that doesn’t overstretch or over-stress you. Record your progress to confirm for yourself that you’re getting ready.
And that’s it. There’s no trick, just you doing the work.
Or maybe, if there is magic, it’s all you.