Keep it to a Minimum
Humidity that is low is best around 30-35% depending on where you live and where your dragon's enclosure is located in your home you can do different things to change your humidity levels. You don't want to have the humidity too high this can cause respiratory issues. Humidity is essential to dragons mainly because is aides is shedding.
Know your Stats
In order to know if your humidity is the right level you need a meter. There are many choices on meters and which you choose is up to you, but you should have one that monitors both temperature and humidity.
There are common ways to keep the humidity at the right levels. You can mist your dragon with water a few times a day, which will hydrate them as well. Be careful not to over do it, you do not want the cage so moist it fogs up as this can cause respiratory issues. Also a lot of owners keep water dishes in their dragons enclosures, which is a good practice especially with baby dragons. Keep in mind with water dishes they need to be cleaned daily as they will bathe in them and often defecate.