I Cleaned the sand first, then removed the fish and saved as much as the tank water as I could, then dumped the rest of the water.
Then once that was all done I removed the gravel and gave the tank a real good clean with tap water, then emptied the dirty water.
Then I put my sand in and levelled it off and placed a plate on the sand while starting to fill the tank (with the water I saved) letting the water go on the plate first so it dont disturb the sand too much. then filled the rest of the tank with warm tap water to make it as close to my tanks normal temp. then added the fish. job done