I made a slide-show, hope I didn't miss anything except we poured a gallon into each tank, Black and both Grey
I used a bucket to remove all the yuck from each tank and poured it into our stool. Not a feat for the weak stomach.
We had dumped thanks, but there was....stuff left over in them.
On edit, make sure you turn off the water heater black rocker switch before you drain the heater if you are hooked up, also be careful of the hot water if hooked up and draining.
Hope this helps;
http://tiny.cc/Winter74