you should define first what absolute and what long term stability and temp. you need. easiest is a bandgap based voltage reference, depending on your answers there are then different options.
secondly, you need to define what volatges you calibrate. even if you have a dmm that does artifact calibration (it calibrates itself based on only one voltage) you then need to verify the defined tolerance is met in all ranges. for that you need to have high and low cal. voltages. typically, this is done by generating a vailable voltage and using a precision self-cailbrating voltage divider. then you can refer all the voltages to the one bandgap reference.
and then you also need to define of you want to also calibrate ac-voltages. if so, it gets more complicated but is doable in principle.