I'm not super tech savvy with this stuff, so pardon my terminology in this video. The 1/8" output needs to be TRRS in order to work. Most converters you buy are only TRS, which won't work with an iPhone. The one I'm using is called "Peterson Icable Guitar to iPhone/iPad/iPod Touch and Android Adapter Cable".
I bought another converter that's supposed to do the same thing, and it did not work, so I'd just stick with the Peterson one.
Here's the link to amazon: Get it on Amazon