Welcome to BYC, I hope you enjoy your time here.

There are many questions coming your way: First please read up on incubation and hatching with Sally Sunshine
https://www.backyardchickens.com/th...g-w-sally-sunshine-hosts-bantychooks-and-more
Type of incubator being used? Do you have calibrated independent hygrometer and thermometer that you calibrated yourself so you will be able to know exactly what is going on inside your incubator?
Age of eggs, fertility of eggs, etc.
As to how to monitor and maintain humidity, usually with trusting the information that your hygrometer is giving you and then adding water to the bator if needed. Candling your eggs and monitoring the air cell size is an excellent way of understanding what your eggs need. Good luck with your hatch but please go into it armed with as much information as you can find because it is heartbreaking to lose babies because of a preventable mistake.
Keep us posted with your progress please. Hopefully some of the duck folks will be along to add information for you.
@WVduckchick you know about these!