You make a humidity box out of something like a rubbermaid bowl. Cut an access hole in the lid or on the side and fill with damp spaghum moss (some people have had success using just a damp paper towel.
You can leave it in all the time or just when in shed. Making sure to check when it needs to be rewet. You don't want your snake laying on damp substrate too much - leading to belly rot, etc.
However, looking at the snakes you have (based on your signature), you shouldn't need one - unless your snakes are having problems giving complete sheds.
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