Litter box training rabbits is not complicated. Just as cats, rabbits gravitate naturally to the litter box without much guidance from us. This is one of the reasons why rabbits make such great pets - they are typically very clean animals.
Cats will use their litter boxes differently than rabbits. They do their business and get out. Rabbits will often use the box as a nice place to nap, eat, stretch out or play (if they are the type to dig).
Rabbits are grazing animals. Have you ever seen other grazers such as cows or horses out in a pasture? Most often they are eating the grass - and sometimes - you can see them poop at the same time. Rabbits are the same way - they will eat and poop at the same time. That is why putting hay in the litter box is a very effective way to litter box train your rabbit. They will just jump in the box and start eating and pooping. Keep in mind, rabbits will NOT eat soiled hay, so it's important they constantly have fresh hay in their box.
There are certain circumstances in which no amount of training will be successful. Unspayed or unneutered rabbits, large groups of bunnies or seniors and those with special needs will most likely have deplorable litter box habits. In the case of unneutered or unspayed rabbits and those in large groups, the reason is territorial - much of the poop and urine outside of the boxes is territorial marking. Spaying and neutering, and separating a pair or trio from a large herd, may improve the cleanliness for the smaller group. For seniors, injured or special needs rabbits, they may be unable to use the box regularly due to physical or medical limitations. If a normal healthy bunny starts to urinate outside the box, I would have them immediately checked out by a vet, as that could be a sign of an illness.
Training can involve a couple of different things. For my first two bunnies, Whoppy and Oso, I had a few litter boxes and then watched where they liked to urinate. Most bunnies prefer to back up in a corner and relieve themselves there. Training was no more than sticking a box in their favorite corner. In the beginning, they switched favorite corners, but after a couple of weeks, they settled into a routine and I was able to remove the extra boxes.
Intensive litter box training involves lining a pen completely with litter boxes and confining the bunny to that pen. Eventually, they pick a corner and you can gradually take the extra litter boxes away.
You can extend this training, too, by putting extra litter boxes in your house in which the bunny is allowed to roam (i.e. if the bunny lives in a room and he is allowed to exercise in the hallway, place several boxes in the hallway). I would not allow the full access to the exercise space right away during this training session. Gradually open up the space as the litter box habits improve. So if your bunny uses his box in the pen, you can open it and have a couple of boxes in the room. If that goes well, you can open part of the hallway, and if that goes well, open more of the hallway. Once your rabbit uses one or two boxes exclusively, you can take away the extra boxes.