When I first got my original pair of buns, they were babies, so I litter trained them on my own. I read that you should treat the rabbit's cage/pen and box as his own territory - no picking him up or putting him back in, just allow him to come out and shoo him back in nicely when play time is over. That way he will realize that you respect his space and he'll be less inclined to mark "your" territory outside of his area. Also, never scold accidents like you would for a dog, and don't put the bun into his litter box/cage as punishment; he may start to associate his box with punishment and therefore not want to use it! It is also a super helpful to gradually increase his play area so he will always remember where his box is, and to use a few boxes once the area is large enough. Another thing that helped me is using a food dish that fastened to the side of the cage. I put it right by the litter box so even while eating pellets my buns could poop and it'd go in their box. The dish I got was pretty cheap and broke quickly, but after they were trained, they didn't poop on the floor while eating from a normal dish. Following these tips, my two never urinated or pooped outside their box, even when visiting a new place! Which made for a very happy bunny mom. Hope this helps! :goodluck