Understanding how you use a hammer and dolly in conjunction with each other takes a lot of practice. You have basically two different methods when using them together. “Hammer on dolly” and “hammer off dolly”.
The “hammer on dolly” method is hitting the metal with the dolly directly below the impact point. When hammering “on dolly” the metal slightly stretches. If you are not careful and hammer to much on the dolly you can cause the metal to over stretch which can cause problems. You will then have to learn how to shrink the metal to get it the back to it’s original shape. You can also over work the metal causing it to “work harden”.
The “hammer off dolly” method is using the hammer to hit a high spot down while the dolly is held on a low area close to the high spot. The rebound action from the hammer blow will cause the dolly to push up the low spot which makes the metal “even out” or straighter.
The best way to learn before you go hammering on your pride and joy is to find an old junk fender to practice on. If it doesn’t have any dents on it make a few with a large hammer and then practice away.

Ron Covell has a great article on the MetalShapers.org about hammer and dolly techniques.
Go HERE to read the article.