The simpler would be a resistive divider as suggested above. It's safe. Benefit is that it gives you a pull-down for free when the "12V" signal is floating.
Now since you mentioned how you get this 12V signal, we know that speed is not an issue. But, a resistive divider as is would be, as the signal comes from a pull-up to 12V (is the pull-up value known and purely resistive?)
If it's a known resistor value, then all you'd need is a pull-down resistor, which will act as a resistive divider combined with the pull-up resistor. Done. You could add a diode to Vdd (3.3V) to clamp it, just in case something goes wrong.