I have been looking at Continuously Variable Transmissions, CVT(s) and Infinitely Variable Transmission IVT(s) for a few days and decided to make myself one.

i will walk you through the key parts so you can do it too
(please give constructive comments about anything in this instructable...especially about the usage of terms)

This is a plate and wheel model, basically the IVT part works by moving the wheel nearer or further from the center of the plate. The nearer a point is to the center of the plate, the slower it moves and thus turns the wheel slower but with more torque. The CVT part is the fly wheel which "contracts" along its axis when spun and thus, moving the wheel further from the center of the plate.

due to some unknown problems, i am unable to add photo notes or embed my video, please bare with me while i try and fix this problem

oh, by the way, i am in the 13-18 category.

Alright, as pointed out by fellow user jj.inc:
"Actually, just for informational purpose, this is a CVT only, not an IVT. An IVT can output at such a low gear that there is no output at all. A ∞:0 ratio is not possible with this device, so it is a CVT which from ratio A:X to ratio B:X where X ≠ 0 can have infinite variance."

learn more about CVT and IVT by just google-ing it
http://en.wikipedia.org/wiki/Continuously_variable_transmission
http://auto.howstuffworks.com/cvt.htm

Step 1: Plate and Wheel Assembly

Just attach a CD or any disc you are willing to sacrifice to the Lego parts ensuring that the lego axis is dead center.
i use wheels with tires to get a good grip on the CD, it might be better to make holes in the CD for the Legos to attach better

next you would want to get a wheel with its tire as the driven wheel, make sure you use one with good grip, the bigger the wheel you use(diameter) the slower it goes, but with more torque

Step 2: Flywheel Assembly

Here you would want the output shaft to be stationary while the shaft connected to the plate and wheel assembly to be able to slide in and out. The flywheel uses 3 axis on each side to make it able to "contract" when centripetal forces move the middle axis out.
Remember to add a rubber-band or two or the flywheel might not return to its default configuration, selecting the rubber band requires some trial and error as there are many variables.

Step 3: Powering the Unit

Since 99% of this project is made of Lego, i would use Lego to power it.
here i used two 5000rpm lego motors with very very low torque, so i geared it down a little (1:3 i think)
but feel free to use anything you like