Factory calibrated Fluke is a good start.(G) Typically calibration is pretty simple. Connect meter to calibration source, turn trimpot inside meter until the numbers match the calibration source, generate the paperwork while sealing the meter back up.
a reliable calibration source is the tricky part for us cheap
folks who don't want to pay Fluke or others the $$$ for calibration. Using a "reliable" battery
(i.e. fresh silver coin cell) or measuring voltage drop on a reference diode, etc. all are a bit less reliable, and if you really want to test the "20.00" volt scale, they're just not a good fit.
I broke down and made a ten-volt reference source a few years ago. A precision source, a couple of bits, and of course, the crucial miniature Altoids tin to house it all.(G) Any time a meter doesn't say "10.00" when it is hooked up, I know there's a problem. Net cost, about the same as the two-way postage to send one out somewhere.
You want some fun, compare a Horror Fright "free with" meter to your Fluke.(G)