To calibrate your hygrometer you will need table salt, a see through container such as a Ziplock baggie, and a small shallow open container, like a bottle cap.
Place a teaspoon or so of salt in the shallow container and add a few drops of water to get it wet. You don't want to dissolve it, just get a good damp pile of salt in your container. Remember, you don't want a salt water solution, just damp salt. Place the shallow container in the baggie along with the hygrometer.
Seal the baggie with some air trapped inside (so it is not tight against the hygrometer) and let it sit. Allow this to stabilize for at least 6 hours (don't rush it). After it has stabilized, check the hygrometer reading without opening up the baggie. It should be exactly 75%.
If your hygrometer doesn't read 75% after calibration then you have 2 choices. First you can remember the deviation of y our hygrometer. If it read 80% then you are 5% off and have to subtract 5% from whatever it reads. The second option is to adjust the hygrometer if your model allows for this. Some hygrometers have a dial on the back or side that can be turned to adjust the unit to 75%. If yours doesn't have this, then go with option 1.
