I clean mines in a basin in the sink and have the tap running and continually moving the sand around and emptying out the basin until the water is clear.
Better is to do it all outside so you're not clogging your pipes with sand! I fill a 5g bucket about half full with the sand, turn on the garden hose and just keep poking it in the sand until the water runs clear. Then I do it some more. Then a little more. I'd rather spend the time outside making sure it's clean than to not take the time and find out it's not clean enough in the tank!
Much what the others have said. I didn't clean mine out enough and the water was cloudy for days afterwards. It's not a massive deal as the filter will deal with it, but do your best to get it clean as you can beforehand.