Don't let these guys scare you the only way you are going to learn is by trying. As long as you a patient person and have some time to put in to it you will be impressed with how it turns out.
Definetly use a sanding block that was my mistake when I first did it.
Get the bondo with fiberglass in it, otherwise it seems to have a tendency to crack. when you are cutting the metal away, feel the back of the metal for the rust, keep cutting until you don't feel it anymore. A 1" spot of rust on the outside probably means a 5" patch of rust on the inside. A dremel is very handy for cutting the metal away, controlable a leaves a nice edge.