How to convert 18 volts to 5 volts?

I have a 18v drill battery that I want to convert to a iPhone 4s charger (5v) as a backup power source. The drill battery is light weight and holds a long charge and I already have it, so it would be ideal. I also have solder and a spare iPhone usb cord. If anyone can provide me with a Radioshack parts list and some instructions Id appreciate it.

You can just buy an emergency charger for your iPhone. It would be cheaper and less time consuming… Unless you're dead set on doing a DIY project, in which case it would be better to go with what Dylan said.

18V to 5V!

Use a voltage regulator which is called " voltage regulator 7805"
then solder it on a protoboard/ dot matrix board with capacitors on input and output!
don't forget to add a resistor and an LED, just to make it looks like a cool stuff!

To convert 18 V to 5 V, you must use the voltage regulator which directly converts the voltage. But you have not mentioned that it is either ac voltage or dc voltage. Otherwise you are advised to use the step down transformer which converts higher voltage to lower voltage.