Most humidors come with a humidifying type of equipment. It could be round or rectangular that you fill with solution, or it could be battery operated or a plug in electrical unit. It depends on the maker of the humidor, and the aftermarket items you want to use.
Remove all packaging from the humidifier and fill it with a quality propylene glycol solution or distilled water. Be sure not to over fill, it should be wet, not soaking or dripping. Sometimes it helps to place it in a water bath and allow it to fill up. If it is of good quality, it should feel heavier when filled, and a lot of the water will be soaked up out of the container.
Wipe any excess water off the unit and place it in the humidor. There may be magnets, velcro or other sticky device to attach the humidor, or it may simply lay inside the unit.