Dilute the muriatic acid with water to lessen its strength. You want to use only enough to remove the cement from the stones. Any more than that runs the risk of damaging them.
This is important. The order in which you mix water and muriatic acid is important; these steps are not interchangeable. If you haven't yet, read this article on safety procedures when using muriatic acid.
Put some clean water in a five gallon bucket. I usually fill up about a quarter of a bucket. Then, carefully pour in muriatic acid. Start with a small amount of muriatic acid and move on to the next step. If you need more muriatic acid, carefully pour a little more into your bucket.