I used a sheet.
I used a sheet for mine with my first, I had 2 so if they are sick was easy to replace. Didn’t want to ruin the mattress if they were sick or something on them
Could use a folded muslin cloth underneath baby’s top end folded under the mattress incase there’s any sick/dribble as easy to wash x
I used a Moses basket sheet with my son and also put a Muslin under where his head would be in case of any sick (saved the sheet getting covered and just replaced the muslin) x