The center of the page is what you should pay attention to (the stuff on the left is just to be able to modulate an exterior power supply, plus power supply caps and diodes and all that stuff you don't need me to tell you about). Note that instead of tying the capacitor straight to ground, I instead tied it to a potentiometer, with the center pin attached to another capacitor which is tied to ground, and the opposing pin also tied to ground. What this did is essentially allow me to selectively put the second cap partially or fully in series with the first, which will decrease the total capacitance. If the center pin is swung all the way to the ground terminal, however, both sides of the secondary cap will essentially be tied to ground, removing it from the circuit and allowing the full wavelength that the primary cap would otherwise provide. Because you are only changing the capacitance, the duty cycle will be unchanged, granting true independent duty cycle and frequency control.
NOTE: this is a placeholder. The full instructable, with photos and a board layout, will come as soon as I have time to finish it.