SGM, I learned the technique in a thread here. You take a cup of water with salt (don't ask me how much, I just dumped alot in). Put the cup in a heavy plastic ziploc. Please the hygrometer in there. Zip up and leave for 8-12 hours. After that time period, the hygrometer should read 75%.
Mine read 70%, and since I did not know how to adjust it, or even if this model allows that, I jsut add 5 when I adjust my humidity.
Hope this helps. It is really important to calibrate, as you could be having too high or too low humidity.
Good luck!