I drain and fill at the sump, (the sump is outside), as it dont matter where the water comes from it is all the same...
I usually clean the tank, wipe the walls down, scoop up somesand pellets or other dterius that I see layaround, and when teh stuff is in the water column, I drain water from my sump, (About 20-30 gallons). This is a bit more then my sump can handle, so I have to turn my return pump off, (which I clean and put back online after), then close the drain valve, then open the WC valve, and add back water to the sump and after about five gallons, I turn the return pupmp onk, then the skimmer, until water is replced...
The other things I do ischeck the pH of the new water and the salinity, although I checked the salinity the night before... I also turn off the Top-off water sensors...
I change the filter socks after the water change, (About a half hour or so), to make sure I catch all the crap inteh water column)
my tank is 180 andthe sump holds 55 but has about35 in it