Seasoning Your Humidor
All humidors should be "seasoned" after you buy it (or if it's been sitting around unused for any length of time). It's easy enough to do, so make sure you take the time.
Just take a moist towel and wipe down the inside of the humidor with distilled water. This can also be done with a spray bottle, but take it easy and don't overdo the wetness. This will remove any dust that may have found its way inside. Although some "experts" don't recommend this process, it is the generally accepted of helping the humidor to season faster.
The Manual Method
The final step is to set a container of 50% distilled water and 50% propylene glycol inside your humidor and close the lid. After about 12 hours, check. If the water is gone, or almost gone, repeat the process and leave the water container inside for another 24 hours. It may take a week or two for the wood to absorb all the moisture. When the liquid stops evaporating, you're good to go and you can place your cigars inside and close the lid.
The Semi-Automated Method
Humidor Humidifier
Another, easier way, is to buy a humidifier for your humidor. This is a little device that you fill with distilled water which will do the same thing as using a container, but with no chance of accidentally bumping the box and spilling the liquid. You also can't overfill a humidifier as it will only hold so much liquid. After you fill it, turn it over a lightly tap it to get rid of any "loose" drops of water. Then, every 3-4 days add more water to the humidifier and spray the box with distiller water once or twice more.
Keep an eye peeled for mold inside your humidor. If you spot some, take action immediately or you will ruin whatever cigar collection is in the humidor. The propylene glycol will help act as a mold inhibitor.
Chances are that when you buy your humidor, there will be directions with it that explain the best way to season your particular humidor.