By not covering the holes in the top and/or sides of its container with saran wrap. Some used to say that putting saran wrap on the air holes was great because it kept humidity in, and while it does keep moisture in it allows disease to fester and kill your spider. Just mist the cage for humidity, never plastic wrap its air holes.
Ventilation is far more imporatant than humidity. I think humidity is overrated anyway. So worry about the ventilation first, no plastic wrap or anything. Just make holes in the container or buy a proper screen lid for the proper sized tank.
What kind of enclosure do you have ?
Ventilation for arboreals is real important especially if your misting. Without air flow you will have mites, mold and other funky stuff growing in there along with your T.
This was made using a pretzel container. I used a hot poker and put in some hole. All my arboreals usually build there web tubes at the hole line.