You will need the following:

• Digital Hygrometer (don't bother with analog)
• Pop or Water Bottle Cap
• Table Salt
• Water
• Large Ziploc Bag

When ready, follow these steps:

• Fill the bottles cap with table salt
• Wet the salt with water so that it feels like damp sand (do not dissolve the salt)
• Carefully place the cap (filled with salt and water) and the hygrometer in the bag and seal shut
• Wait at least 6 hours
  

If your hygrometer is reading under or over 75% at the 6hr mark, you’ll need to either adjust the reading or note the deviation. Some devices allow you to change the reading so that it displays an accurate humidity level. However, most don’t so you’ll need to remember the deviation and add or subtract the difference when reading the display.

You should take the time to calibrate your hygrometer at least twice a year (summer and winter).