(Or, at least, here’s how I learned how to juggle. Roughly.)
1) Start with one tennis ball in your dominant hand. Practice throwing it to just about eye level with gentle toss, and try and catch it in your non-dominant hand without moving your shoulders and limiting your elbow movement — imagine you have a hanger in your shirt. Do this until you can make the release and catch consistently — ten in a row a few times.
2) Repeat Step 1 for the non-dominant to dominant hand.
3) Now, take a tennis ball in each hand. (Two total, to be clear.) Repeat Step 1 but, when your non-dominant hand makes the catch, practice tossing the ball that was in that hand upward to about eye level. You don’t need to worry about catching the second ball yet, just get the (in my case) left hand exchange down. You should be doing the exchange in one motion — when the left hand tosses its ball upward, it grabs the other ball at the peak of the tossing motion. If you’re doing it right, both balls are only in the air for a fraction of a second. Again, ten consecutive successes and you’re probably good to go.
The trick here: when your make the exchange, toss the second ball from a position inside of where you are going to catch the first ball. Like, imagine you had a small umbrella open inside a bigger umbrella. The first toss should follow the path of the big umbrella; the second toss should follow the path of the small umbrella.
4) Optionally, repeat Step 3 but starting with your non-dominant hand. It’s not all that important, honestly (I’m not good at it and I’m a pretty solid juggler), but it’s not bad to do while learning.
5) Okay, now repeat Step 3 but now you need to make the second catch to get credit. Ten in a row should mean you’re there, but again, use your judgment.
6) Optional: Put two tennis balls in your dominant hand. Repeat Step 1, but only throw one of the two balls. This is how you’re going to start any juggling routine, so you need to make sure you can get a good toss here. For people with bigger hands this is going to be really easy; others may have to practice.
7) Start with two balls in your dominant hand and the third in your weaker hand. Toss one of the two balls in the strong hand, catching it with the weak, and as the weak hand makes the catch, toss its toward the dominant hand. And this point, you don’t need to make the exchange again with your dominant hand — that we’ll do next. Again, do this until you succeed at it ten or so times in a row.
8) Now, add in the catch-and-release on the return. If you’re a righty, that means you toss a ball from your right to your left (while having another in your right), catch that with your left while tossing the second ball back to your right, and finally catch that second ball with your right while tossing the third to your left. Keep making that exchange over and over again for as long as you can, and congrats, you’re juggling.