Add several drops of essential oil to the water tank of a humidifier only if you don't mind replacing the machine more often. Some humidifiers get around these drawbacks by adding a special mister and fragrance diffuser that holds the oils separate from the main tank and mists them into the air.
You most likely have a top-class room humidifier in your home already, so you can add essential oils in humidifier immediately to benefit from their effects.
Simply add 4-5 drops of Essential Oil (or, your favorite blend) to your diffuser, add distilled water, and turn it. Can you diffuse essential oils in your humidifier?