Bam!
Done.
Why go through all the hardware hacking when there are off the shelf products that do exactly what you want?
I'm not sure what you mean for your papers, though. If you're planning on doing this for a school project or something, all you need to do for that is to wire in a thermostat in series with a resistor. When the temperature exceeds your preset level, the thermostat trips and bypasses the resistor, which causes the fan to speed up. You could in theory use multiple resistors to create stepping, or even a variable one if you want finer control, but the simplest circuit is one thermostat, one resistor, one fan.
No diagrams. If you really want a wiring diagram, draw it yourself. I've given you everything you need, and if you don't know how to diagram it you shouldn't be doing something like this in the first place.