Once the tank is drained and the old valve ball is removed, there will still be water left in the very bottom of the tank between the tank bottom and the top of the flush valve seat. This water needs to be removed and the easiest way to do this is to use a sponge. I've found a wallpaper sponge works pretty well. If you don't have one handy any sponge will do.
Sop up the water, squeeze it out into the sink and repeat until the tank is fully drained.
