Cool, but yes, drain your tank as much as possible and take the inhabitants out.
Because you are doing it this way I would wash the sand even more thoroughly, then you can just dump it on, smooth it out, and it should settle quite quickly, then fill the tank up with a hose (if you dont have the means to store the old water), and get a heater (and any spare heaters you have) working on it straight away), by the time the tank is up to temperature, the sand shouldve settled, then you can turn your filter(s) on and re-add the fish