So if your new to worm bins watch your moisture levels. If your a Worm Inn bin user this is especially true since this system is open and is allowed to breath. By putting a cover layer over the top like carbon materials like news paper, cardboard egg cartons, shredded junk mail(not the glossy type or any plastics), leaves etc you are mimicking a forest floor mulch that the worms can thrive in. This mulch layer will help regulate your moisture levels.
Here is a video on worm bins that you might find helpful:
Take care, and happy worm farming! :-D