With rabbits, litter training is mostly about the urine. Only the rare rabbit will be 100% with poos. But that shouldn't be a worry as the poos aren't an issue -- dry and odorless. Plus they make excellent fertilizer! It is the urine that s m e l l s strong! That's where the stink comes from.
Rabbits naturally will go potty in the same place. Starting with adults (as you said you were) will make it easier (believe it or not) because they litter train more readily than babies. Use a good size plastic tub or tote. Wood pellets (not shavings) work great for absorbing urine odor. I topped mine with hay. The hay entices them to go in there. They like to potty while they graze. Don't worry. They won't eat soiled hay. I just add fresh hay on top each day.