and slowly pour out most of the water, being careful not to pour out the gravel.
place a pan or something to catch the gravel near the tank and pour out the gravel. if the tank is too heavy to turn over, you will need to scoop it out.
set 1 or 2 cups of gravel aside and don't clean it. waste eating bacteria has more than likely developed in the tank and saving a bit of the gravel will keep this bacteria alive so that it can resume its duties when the tank is put back together.
use a hose to rinse the gravel. fill the pan with water and use your 