What Ways to Calibrate a Multimeter?
Very generally and at the most basic, you need a precision voltage reference and resistors.  Obtaining 0.1% is fairly easy, for resistors it's straightforward, and for voltage references look up "precision voltage reference" (something like this), and bias it with 0.1 to 1 mA.

The actual calibration procedure will vary from meter to meter, so you will need to dig up the appropriate documents.