The basic principle is a capacitive divider - just like a resistive divider except that the formula is C1 / (C1 + C2) rather than R2 / (R1 + R2) for the division ratio. Thus if the pickup probe is 1pF (for example) to the HT lead and you want 0.0001 ratio, C1 = 1pF, C2 = 10nF and a 50kV pulse will appear as 5V pulse across the 10nF - both capacitors take up 50nC of charge.
In practice you'll have to calibrate the pickup probe yourself
You also need protection circuitry such as a string of high-voltage series resistors inline with the probe and a zener diode to ground to prevent high voltage transients escaping into the output.
Also remember modern car ignition systems with capacitive discharge HV generation can easily kill - make sure all the HT leads are clean and in good condition (no cracks).