Litter training rabbits is fairly easy, because rabbits already centralize their urine and feces into a single place (usually the corners of the cage). The best method is to get a corner litter box and every time the rabbit defecates elsewhere, you should move it to the litter box. Filling the litter box with a separate kind of bedding (but not cat litter!) will also help your rabbit to quickly learn where it should be going to the bathroom.