The hygrometer is the only thing that you can count on to make sure the humidity in your humidor is ideal so be sure to do this step!

Take one tablespoon of salt and place it in an open container.  The lid of a jar or even a small cup will work fine.  Start by adding about half a teaspoon of water to the salt.  Stir it in and make sure you put just enough water to wet the salt but not dissolve it, at most it should turn into a thick paste.  Next, place your hygrometer and the salt-filled container into a zip-lock bag.  Seal the bag carefully, making sure not to spill anything out of the container.  Also, be sure to leave some air in the bag so the hygrometer can do its job and read the moisture content in the air.

After about 6-8 hours, check your hygrometer.  Ideally it will read 75%, however, most hygrometers have a certain margin of error.  To compensate for this you can either remember the margin of error (if it reads 70% you know it will always be off -5%) or if your hygrometer allows for it, adjust the reading until the needle reads 75%.

If you've got an analog hygrometer, there should be a screw on the back of it.  You can turn that screw until it reads 75%.  Some digital hygrometers can be adjusted to correct your reading.  Otherwise, you'll just have to remember the margin of error when you take measurements.

You're ready to charge your humidor!