Chemicals can be mixed and stored ahead of time.
Make sure to squeeze any air out of the top of the datatainers, to help preserve the chemicals. After a few minutes, test the temperature of the chemicals, and make any adjustments to the water in the tray so that it brings the chemicals to the right temperature. You won't need the reel holder inside the tank, just the tank and its lid. Let your eyes adjust, and look for light leaks. Fix any before proceeding.