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).
I'd be cautious then, those high voltages don't take prisoners and protection circuitry is important. There might be an existing meter that you could interface too at the low-voltage side - have you details of your handheld meter to share - there might be a way to connect to it that would be simpler and less risky.