Firstly, why and when should be a guitar humidifier used?
As all we know, Safe humidity environment for the wood guitars and ukuleles is 42%-65%.
Relative humidity from 0% to 25% is called "DRY" and from 25% to 40% can be called low humidity.
Short-term exposure(about 3 days) in the environment from 35% to 42% is OK. If a long time, it may cause problem to the guitar.
By using a humidifier, we can keep the relative humidity in the safe humidity environment(42% to 65%).

Now let's  introduce about how to use the humidifier.

1. Use dropper to feed water through the hole on the humidifier.
    If no dropper, we can put it under the water tap directly.
    We need to be pay attention that water can't be overflowed from the humidifier.
    The surface of the humidifier should be dry when we put it into the guitar sould hole in case of making the guitar wet.
 
                          
                           
2. Put the humidifier into the guitar sundhole. 
                                           
The guitar should be put into the guitar bag or guitar case and we should check humidifier from time to time to see if needing to feed the water or not.