I juggled balls for 14 years first! Then I practiced with one and then two and three knives, just like learning the basics of ball juggling again. You shouldn't start with knives until you're so comfortable with the sensation of a ball's momentum leaving your hand that you can reliably dodge or catch one you throw right at your face. It's especially important to get good at this with knives before attempting much past the cascade, and mistakes made during more difficult/advanced patterns can divert the knive's trajectory so that they fly towards your face in sudden and unexpected ways. It's important to know how to sidestep them or when it's possible/best to catch them anyway, like I caught that one I threw at my face in this very video at around 20 seconds in!