Cut one end of the USB cable off. You may wish to give yourself as much cable length of the male side of the cut cable. there are 4 colored wires inside the large wire. expose the red and black ones. these are the power wires that would power the USB device. the white and green ones are for information transfer and aren't used.
History Lesson: A USB cable puts out about 5 volts of power, just enoguh to power most LED's and battery operated Christmas lights (they usually take 2 AA or D cell batteries which are 1.5 volts a piece. 2 batteries = 3 volts, so a USB cable has more than enough power but not too much)