For example, I want to cut a hole in the car's front left fender, to fit a snorkel.
What is the best way to prevent rusting on the exposed metal on the edge of the cut?
Sand down the paint around the hole, apply a few layers of zinc spray followed by a few layers of new paint (with primer, if necessary) with 1 or 2 layers of clearcoat on top. Make sure you cover the edges well and check the edge regularly to see if any rust occurs.
Make sure you wait long enough between each layer for optimal protection, check the label for detailed instructions. Don't think one layer of each is enough! Minor damages, like scratches, will go through the thin layers and the metal starts rusting when that happens.