Place a teaspoon of salt in a bottle cap or small cup and dampen it with a few drops of water (without dissolving it).
Carefully place the wet salt and the hygrometer inside a see-through container and close tightly. You can use a zip lock bag provided it seals well and you leave some air inside as well.
Let it sit for at least 6 hours and note the reading on the hygrometer without opening the container. The hygrometer should read 75%. The difference is how much your hygrometer is off. For example, if your hygrometer reads 70% during the salt method test this means that to obtain the real humidity level you should add 5 points.
If you have an analog hygrometer, you can correct the reading. Simply make a mark on the reading immediately after pulling from the bag with the salt solution. For instance, if the hygrometer reads 60%, then take a sharpie and mark the 60% line. This is now 75% and when you read your hygrometer and it's reading 60%, then you will know it's actually at 75%.
IMPORTANT: YOU MUST FOLLOW THE ABOVE INSTRUCTIONS (put salt and hygrometer inside ziploc) BEFORE HOLDING THE BUTTON ON THE BACK - FAILURE TO DO SO WILL UNCALIBRATE A LIKELY CALIBRATED HYGROMETER. If you have a digital hygrometer (built into the front), you can hold the button the back of the unit for 5 seconds to calibrate it to 75%.
If you have a round digital hygrometer, you can turn the knob on the front - 1 turn equals 1%.
Your hygrometer is now ready to be placed back inside your humidor.