Acoustic guitar rentals are often shipped with a Planet Waves Acoustic Guitar Humidifier. To refill the humidifier, open it by pressing on the tabs on either side under the cap, as shown in the photo below. Then remove the cap and remove the sponge from inside the humidifier. Run the sponge under cold tap water and shake off the excess. Replace the sponge and cap. The humidifier should be placed between the 3rd (G) and 4th (D) strings of the guitar.