Android and Arduino

Hello, I've seen that we can program the arduino via android using a OTG, no my question:

Is it easy to combine two USB cables by my own? the arduino and the cellphone one?

If yes how can I send data through the cellphone to make the arduino turn on and off some leds?

Do I need to root my cellphone? (I have a Galaxy SIII and its new so I dont really want to root it...)