How do design a high voltage voltmeter?

Use a voltage divider and an standard high input impedance voltmeter connected to the low voltage output tap of the divider. Just check that:

a) the voltage divider has enough resistance to minimize loading of the voltage source (The voltage divider resistance should be as high as possible).

b) the voltmeter's input impedance is at least 10-20 times larger than the output resistance (impedance) of the divider. If necessary add a high-input impedance amplifier or a transducer between the divider output and the voltmeter.

What is high voltage for you?