Here is a good website for quick reference, also has list of whelping supplies, and other info. I also agree that you should call your vet, especially if you have a breed that has a difficult time whelping.
If your dog is ripping and tearing at her bedding, she's ready. Most dogs don't have a problem ,except for the over anxios "grandparents"(depending on your breed). When you see your dog"laboring" with no sign of a pup, you have a problem. I am always on hand, with scissor, towels, alcholol, etc on hand. 'DON'T interfere if you don'y have toooo!" I have had pups be born 5 hrs inbetween. Just be ready, and have your vet on standby!