Wait until your rabbit picks a corner of the cage to pee in. Then, put the litter box in that corner. When she goes outside of it, scoop it up, put it in the box. Poop is harder to train, but over time, it will get better.
I trained my rabbit exactly like you would train a cat, I put her in the box. Rabbits like some privacy, so the box should be in their cage and always open to them. Clean it out a lot, they constantly go. Be prepared, they will have a lot of accidents.
Keep the litter box clean or the rabbit will avoid the area. Place the rabbit in the box; when he/she voids, let it out to play; repeat until he/she learns.
