You prepare by fucking up a lot. You're gonna have to repeat this shit experience enough times that you get numb to it.
If you want to fuck up again, but in a more controlled environment, maybe check out some mock interview service like interviewing.io or others.
But, that's how it is. I just went through 7 months of getting punched in the balls before getting lucky.
(Also, leetcode and general prep does help a little too.)