2 litres apple juice (cloudy looks nicest, but whatever you prefer)
1.25 litres ginger beer
1.25 litres soda water
4 Granny Smith apples, cut in wedges
4 lemons, sliced
This could not be any easier; put it all in your punch bowl/jug/drink fountain/liquid receptacle of some kind. If you want to prepare ahead of time then put the cut fruit in a container, ensure that the apple is coated in lemon juice to keep it from browning, then pop the container in the freezer. This also keeps your punch cold for longer without watering it down too much with ice.
Throughout the party keep topping up the punch with the same ratio of liquids and you'll have all your guests asking "what's in the punch?" and wanting you to write down the recipe.