Use an accurate voltage source.
Fully discharge the capacitor by shorting it to the reference common. Keep the short applied long enough for the effects of dielectric absorption to dissipate.
Apply the reference voltage, and measure the time required to reach (1-1/e) times the reference voltage (approximately 0.63212 Vref). An accurately calibrated comparitor is useful for this.
Scale the resistor value to achieve the desired Time constsnt.
Your main problems will be:
Capacitor variation with temperature.
Capacitor insulation resistance decrease with temperature.
This can be mitigated by choosing R << Rinsulation.
Capacitors with the lowest temperature coefficient are:
Polystyrene (Only good for temperature <85 deg C)
Teflon (Expensive and physically large)
Polyporopylene
Polycarbonate (Smallest physical size, but as good as above)
NPO or CGO Ceramic (Low temp coeffecient, but high dielectric absorption)
Avoid Tantalum, Aluminum. They age, and have high dielctric absorption.
Added after 2 hours 26 minutes:
Correction to my previous reply:
The statement Polycarbonate (Smallest physical size, but as good as above) should read Polycarbonate (Smallest physical size, but NOT as good as above)