1: Make a three stage RC network and an inverting amplifier to go with it.
2: Find the frequency at which the output is in phase with the input.
3: Adjust the amplifier’s gain so that the output is slightly stronger than the input.
4: Make some kind of stabilization mechanism (like a couple of zener diodes to limit the maximum voltage and a resistor to soften the sharp kick of the zener diodes)
5: Hook up the output to the input and give a kickstart (shake the device or add some sort of kickstart mechanism like a button).
How to determine the frequency: Play with the RC network until you have the desired frequency.
The top circuit is what it might look like after the third step. The circuit on bottom is what it might look like at the fifth step.
