I reproduced the instructions below from this post. The original site can take a bit of time to load. Kudos to Tman at puff.com.
How to do a salt test
This is how I do my salt test. Salt has an amazing property where RH% will be pretty much nearly be 75% when it is in a saturated condition with water.
(Look for Sodium Chloride, aka. Salt) http://www.omega.com/temperature/z/pdf/z103.pdf
Here's what you'll need:
Salt (I use Morton Salt)
Small Ziploc© Bag (Small as possible)
Bottle cap (Gatorade cap works best. Wide&shallow works best. I'll use water bottle cap for this)
Toothpick (whatever to mix the salt)
Hygrometer to be calibrated
First, put salt in the bottle cap
Then, put couple drops of water in the salt. Notice it's partially wet. You want the salt to suck up all the water.
Mix the salt/water mix so it will be a good mix.
Once you have the saturated salt, put it into a Ziploc bag with your hygrometer. Pinch up the center of the Ziploc bag so it will inflate the bag. While you are doing this, seal the bag with your other hand. The bags should be inflated so it won't cover the sensor opening of the hygrometer or the bottle cap.
Give it couple of hours for the RH% to stabilize. I think 6 hours should be sufficient time to do this.
Once the RH% stabilizes, calibrate your hygrometer within the Ziploc bag. On HygroSet, I'll have to twist the dial (clockwise for +RH% and counter-clockwise for -RH%. Press the Set button.)
On Xikar hygrometer, press "Calibrate".
It might be easier to use a tip of a pen to press the button. If you want, you can leave the bag for another 6 hours and make sure the reading stabilizes at 75%.
That's it! Enjoy your calibrated hygrometer!