First things first, don't get wires confused! This can equal very bad things. Power supplies can blow (dunno, sounds true), fans can go bad, etc. So be careful, better yet the easiest way is to label them with stickers. Less confusion will equal less headaches later. So here is what the diagram of what will be done :
This type of wiring can be re-done in a couple of different combinations, but this is just a simple example. To start off, the SPDT switch will select either 12volts or 5volts that will get carried directly to the fan. The simplest way I tried was having the switch between the molex connector that is coming from the power supply and the fan bus. From there, you can wire any of the combination of fan spring clips outlets you choose. Remember, this is all customizable!