I have looked on the web and all I have come up with is hopelessly conflicting information on this.
One examples uses a series 10k resistor from a 3V power supply.
Another uses a voltage divider of 3.3k and 5.6k with 9V power supply and no series resistor.
Another uses a 2.2k series resistor from a 12V power supply.
I want to use a 12V power supply so how should I bias my electret?
I don't think it matters much. However, one possibility is to start with 10K and measure the output voltage. Adjust the resistor value until the output voltage is about 6V.
Value of the resistor it depends to the amplifier which you are using for it, for low impedance se low resistor -3k, most microphone amplifiers have high inpedance so Value of the resistor is not important.
If you don't have the specs for the particular electret, choose a resistor (by trial-and-error) that gives you about half the power supply voltage. Or, you could use a pot and adjust to 1/2 voltage.
It shouldn't be super-critical since the signal is very-low relative to the bias.
Don't forget to add a series capacitor to isolate the bias from the signal. And, I assume you know that you'll need a preamp.