You could probably take any cellphone adaptor and convert it into an iPhone charger pretty easy but here’s a cool DIY showing you how to make your own iPhone charger.
All you need are the 4 resistors stated above. I have modified some values of the resistors because of some miscalculation. Connect them as shown and provide the circuit with a regulated 5v supply. Get a female USB plug and connect it as shown to the circuit. Make sure the positive and negative terminals are connected correctly. The D+ and D- pins have low currents so connecting them wrongly may not blow up your iPhone.