The hygrometer is not necessary, but it is helpful to have one. All it does is give you a reading of the humidity level/temperature.
From CigarAficionado: Take a new sponge -- make sure it is unscented and free of soap -- and wet it with a liberal dose of distilled water. Wipe down all the exposed wood, including any trays and dividers, and the interior lid. Avoid using a paper towel or a fraying cloth; these will literally leave a paper trail on the wood. After you've wiped down the wood, squirt the sponge with more distilled water, then place it inside the humidor on a plastic bag -- to avoid direct contact with the wood -- and close the lid.

Next, prepare your humidification device according to the manufacturer's instructions. Unless the manufacturer specifically states that you can use tap water, use only distilled water. Tap water contains minerals that will destroy most humidification systems by leaving deposits that will clog the humidor element. Once the humidification element is filled, be sure to wipe it down to remove all the excess water. Rest it on a hand towel for approximately 30 minutes.

Close the humidor with its humidifying element and the damp sponge, and leave it overnight. The next day, refresh the humidification device (it may not need it) and check the sponge. If it is fairly dry, add more distilled water. If it is very damp, leave it alone.

Let the humidor sit another night, and then remove the sponge and plastic bag. The walls of the humidor have now absorbed all the water they need, and now you can safely store your cigars.