Litter training a rabbit is surprisingly simple as rabbits are very clean animals who always use the same spot as a toilet.
Hutch rabbits / caged indoor rabbits
To make it easier to clean the hutch or indoor cage, you can place a small corner litter tray in the corner that your rabbit pees in. Place some bedding or hay in the tray so the rabbit does not slip when it hops in. This can then be cleaned out every couple of days, keeping the hutch / cage much cleaner between regular weekly cleans.
House rabbits (free range)