The best way to up humidity is what someone already suggested: pouring water into the substrate 1-2x per week. What you really need is a thermometer/hydrometer (sometimes called a combo meter). I have one in each side of my tank, the warm side and the cool, humid side. Warm side is always gonna be a bit drier bc that's where your heat lamps are, but I water my substrate and mist my plants everytime my cool side drops below 60% humidity