Avoid exceeding approximately 50% humidity.
Leopard gecko humidity is quite simple, so let's make this quick! Being from dry climates, they do not need very much moisture in their enclosure, unlike most other popular gecko species.
Keeping it dry generally involves simply doing nothing in most households. If you live in a very humid climate, a dehumidifier may be useful to ensure levels don't get too high. If humidity constantly stays very high, your gecko may become susceptible to respiratory issues, unhealthy skin/scales, or other health problems.
Measuring your humidity is easiest with a hydrometer.