First of all it is quite difficult to litter train your bunnies when they are not neutered or spayed so that would be first on the list, though if you can't afford that right now or whilst you are waiting to have them done I would reccomend finding the corner that your bunnies go to the toiler most in and put the litter tray there. Then pick up as many of their poops as you can and put them in the tray, also if you can soak up the pee with newspaper or tissue you can put that in the tray too.
They will eventually get the hang of it, it could take a while though. For more tips on litter training and litter check out the library