Hi, I think it's important to practice your answers within the given time frame you have (usually 1m30?), so you're used to that going into the VI.
Prepare your answers as you would for a regular interview, only you're talking to the camera and not a person.
This might sound basic but try to sound interested in what you're saying. A lot of us are thinking "ugh I'm basically talking to myself so why bother", but if you come across as enthusiastic then that will never harm you. Smile at the end of each answer
If you screw up, don't panic and then get flustered. Just go back slightly to the start of the sentence, breathe and start your point again.
Speak calmly and slowly! Gives you more time to think about what you're saying, and you won't be so stressed about the timer.
I literally record myself on my phone to assess my tone, my facial movements, where I'm looking etc., so that might be worth doing to see what you can improve on before the real thing.
Hope that helps!