You do aim it so it hits dead on. Then it emerges at some different angle which knocks itself off that dead-on path that you so carefully aimed. Now it's traveling an altered path that makes it emerge at that different angle.
Why does it do that? I get why the principle demands that it does and I get why it's a stable solution once it's been made to happen, but how did it actually make the jump from "straight on" to "at an angle"? Presumably not every self-consistent causal loop materializes itself out of thin air just because it would work if it did.
Edit: possibly lame example, but in Terminator 2, we find out that terminators were a secret government project that went awry. Where did they get the technology? Well, a terminator from the future came back and died in Terminator 1 and the parts it left behind allowed engineers to get a head start on building one. So there's a similarly self-consistent causal loop. But it doesn't really answer the question of how there came to be terminators.
I can't say I fully understand it. One thing I've seen stressed is that there's only one timeline. There's never a time when the ball continues straight through into the wormhole, only to somehow emerge with an altered path. It only travels that path to the wormhole once, and when it does, it gets hit a glancing blow.
The wiki entry I linked is intriguing. It says in some cases there are an infinite number of consistent solutions. They found they could analyze those cases with quantum mechanics, doing a sum-over-histories that only includes the consistent solutions. That gave a probability for each particular outcome.
So maybe that's fundamentally how causation actually works in the universe, and in the absence of time loops it reduces to normal causality. Now I can't help speculate whether actual quantum mechanics involves time loops somehow. (And in fact, Cramer's transactional interpretation does involve waves travelling backwards in time.)
Another way to look at it, maybe, and take with a grain of salt because I came up with it myself: Imagine there isn't just one timeline. In the paradox the universe runs an infinite loop, the ball first going into the wormhole, then not, then it does, then it doesn't. But given quantum mechanics, the position and momentum of the ball is uncertain. Given a very large number of trials, it's eventually going to shift significantly. At some point, it hits on a consistent solution, and the looping finally stops.
