You are shooting for around 75% for hatching. Too low and they can't turn in the eggs and the membranes dry out and make it hard to hatch. Too high (like above 80% or so) and the air cell can partially fill with condensation which can drown the chick when they poke through the air cell.
Fill up the trays according to the directions (use WARM water, about 100 degrees - too cold and it will drop your temps for a while, too hot and you'll have way too much humidity for a bit) Then close the bator up.
Check your humidity and see where it gets after a couple hours. If it isn't holding steady around 75%, then you can add another tray of water to the wire grid - like a plastic storage container, etc - the bigger the surface area, the more humidity you will add. You may have to experiement a bit to get the right sized container.
Once you figure that out and humidity is where it needs to be...
DO......NOT......OPEN.....THE.....BATOR....AT....ALL
until your hatch is complete.
Good luck! Hope you have a great hatch.