If your looking to litter train a rabbit, start by looking at where your rabbit is going to the toilet mainly. Put a litter tray in that place. If your rabbit it going to the toilet in a place you do not want it to, put vinegar there, and this should stop your rabbit messing where you dont want it to. If your rabbit still insists on not using it's litter tray, buy some cat litter and put it into the litter tray, this encourages the rabbit to wee there, as soon as your rabbit is weeing in it's litter tray, take away the cat litter. If your rabbit changes back to a place where you dont want it to wee, put the cat litter back. Keep doing this until your rabbit gets the hang of it!