I want to connect a usb cable to a DC mobile charger of ~5 volts?

Here's how I'm trying to do it:

- I cut the end of the usb cable that connects to the computer to leave four wires namely; red, white, grey & green.
- I am to connect these wires to a mobile charger with only two wires; red(live) & green.

Now this is where the problem arises: I already know that two or three of the usb cable wires are for data transfer and only the remainder are for power, so what I wanna know is which ones are for power and also which one connects to the red(live) and which one connects to the green from the mobile charger.
N.B I already know that the voltage limit or rating of this usb cable and that of the mobile charger are the same and hence there's no need to go to that one, just help me with the connecting thing and I'll be more than grateful.