Plain and Simple - rabbits like to eat and poop at the same time. Since hay is high in fiber and rabbits spend a lot of their time eating hay, it makes sense that one of the fastest ways to litter train your rabbit is to place their hay above their litter box so that they can sit in their litter box and eat their hay at the same time.
The next step is that rabbits can become pretty set in their ways. You'll begin to notice where your rabbit naturally uses the restroom in their living space. It will usually all be contained into one area. The easiest thing to do is choose this place to put the litter box.
Once your rabbit is doing pretty good with litter training in their main living space, you can try teaching them to go when they are out exercising and having some time to free roam. I recommend using a few litter boxes and a smaller space to start with and gradually expanding the space and eventually taking away some of the boxes once you see consistent progress.
Litter training your rabbit is sometimes very simple, especially since they like being clean but sometimes it's not that easy. If your rabbit is taking a little longer to get it, you can try trying different litters to see which one your rabbit prefers. They may not like the texture, or even the smell, of one and you might find better success with a different type. Maybe even a different litter box. You might find that they prefer a larger box or shorter (or higher) sides.
If you still aren't having any luck, and it doesn't seem like your rabbit is going in one spot and seems like they're just using the restroom everywhere, and they're over 6 months, it may help to get your rabbit spayed or neutered since the behavior is most likely due to them trying to mark their territory.