Some solid advice here! I keep quite a few Brazilian Rainbows (over 20 so far), so would add a few notes based on that experience.
Soak the moss as suggested, but afterward ring it out, just like you would a washcloth. The idea is to have it damp, not swimming.
A day after you set it up, re-check the hide to see if the humidity has held. If the top of the hide has condensation when you remove it, all is well. Mist lightly. If not, resoak and wring less water out. Once you get the basic balance down, you should be able to spot clean and mist the hide two to three times a week and be fine.
Screen tops and Brazilians generally do not mix well. They let out far too much humidity to keep this species healthy. If you have only a screen top, purchase a piece of plexiglass to cover most of the top.
Other tricks to raise humidity include having a large, flat water bowl (large surface area equals more evaporation), setting the water bowl on the warm side (promotes evaporation), using an absorbent substrate which you mist periodically, and misting the tank in general a few times week.
The humid hide, however, will always be the centerpiece of your plan, and the snake’s ultimate retreat for humidity.
Good luck, and let me know if you have any other questions.