Without knowing your system or what you want to add, here goes....
Make sure you unplug any equipment that may get wet and make sure its not wet when pluging back in as you do this just for safty.
Mix new water to add to system and have ready because a fuge will need more water than a WD.
Turn off return pump and alow main tank to drain all that it will. May want to place an extra PH in the main tank to keep some circulation if you have one.
Lift out pump and let it drain and set to the side and disconnect drain line from WD.
Remove all water from WD into a seperate clean container and pull WD out.
Place fuge in to place and put pump into pump chamber and drain line securly into drain chamber.
Place in skimmer and any other equipment like UV pumps, Heater, Ect.
Pour saved water into fuge and any new water to fill it about half way and bump on skimmer and anything esle to test and fill up.
Now fill fuge about 3 / 4 full and start system to test for flow and draining and bubbles, ect, add or remove water as needed and let run awhile to make sure.
Start up skimmer and everything and adjust as needed.
Turn off all pumps and let MT drain and then add some live sand, some cured live rock and adjust water as you do this. Add plants and a light and allow sand to settle if needed and restart pumps.
Bioload should be fine and spikes should not happen but test regularly just incase.
