The standard is to go into lockdown after 18 days of incubation. That means stop turning the eggs, position the eggs for hatch, and increase the humidity. You count the days by saying "0" the day you put them in, "1" the following day, and continue until you say "18". A lot of people start counting the day the eggs go in and usually don't have any problems with that even though they are a day early. Timing is not that critical but I like to do it correctly.
Which incubator do you have? What does the floor look like? Different incubators can be different so different things work. With mine I don't want to interfere with air flow so I don't use anything solid. I use a 1/4" hardware cloth section stiff enough keep the eggs out out of the water in the water trays underneath but allows great air flow. Other people with different incubators use paper towels or shelf liner. The chicks need to be able to walk without slipping on a slick surface and injuring themselves.
The idea is to do all of your lockdown work at the same time.
Good luck! Let us know how your hatch goes.
