2-4 crib sheets. And a waterproof matress pad. That is all that is needed. :-)
Honestly. My baby girl is sleeping in a pack and play bassinet combo. Boom done.
I bought waterproof mattress pads as well, just so in the middle of the night after a blow out or something, I don't have to wipe and disinfect the mattress.
Crib sheet. Only thing "necessary".