Explain how natural selection might produce a modern giraffe from short necked ancestors?

Answer 1

Those with shorter necks couldn't get enough to eat, so they died off before reproducing. Those with longer necks that were able to reach the leaves and eat them survived to procreate.

In every species there will be tons of variation. In a giraffe or it's short-necked ancestors, some of those variations will be to make the necks longer, some will make them shorter.

The ones that make the neck shorter will become less common because they will decrease the animal's chances of survival because they will limit it's food supply (because it can't reach higher leaves).

The ones that make the neck longer will become more dominant because those members of the species with the "long-neck" genes will survive longer and reproduce more.

And gradually over time the necks will become longer due to the pressure exerted by the environment (the killing off of shorter giraffes by starvation).
Natural selection also effects the length of the legs, strength of the bones etc, the formation of a sponge-like tissue which prevents the blood from damaging the brain when it puts it's head down and I'm sure other things as well.
Good variations stay, bad variations go.