A brand-new humidor is bone dry. The Spanish cedar inside has not yet been exposed to the humidity it will eventually hold, and if you load cigars into it on day one the cedar will pull moisture out of the wrappers faster than your humidification system can replace it. The result is dry cigars within a week and a humidor that takes months to settle.
The cleanest way to season is the Boveda method. Place one Boveda 84% calibration pack (sold for this exact purpose) inside the empty humidor, close the lid, and leave it for two weeks. The cedar absorbs moisture from the pack until the wood reaches its working equilibrium, at which point your normal 65 or 69 percent storage packs can hold the level steady. Do not use the 84% pack for storage afterward; it is too damp.
The older method is to wipe the cedar with distilled water on a clean cloth, never tap water (the minerals leave deposits). Done lightly and repeated every other day for a week, this also seasons the wood. The Boveda method is more forgiving for first-time owners.
Once seasoned, your humidor is ready. The longer treatment of humidity, temperature, and stability lives in How to Rest a Cigar.