Check to see which valley you will use as your start valley. Make sure that the valley continues the full length of your gathering. If you have one or two extra pleats to the left of this valley you can 'cheat' by sewing them together with the first pleat. This is the easiest way of ensuring that your design will be correct.
This design looks very good using both contrasting colours as illustrated or by using two shades of the same colour. Try to have about two degrees difference between the shades.