i wouldnt like to say personally because im not sure but whatever you do wash the sand well, this will prevent you from staring into a tank of cloudy water for days. what i did is just get a bucket or washing up bowl or something, but i wouldnt wash all the sand at once, you will find doing it bit by bit will be more effective. So, then you put some sand in the bucket, fill with water, swish around, wait a couple of seconds to let the sand settle, but not the dirt that you want to get rid of, then pour out the water, continue to do this until the water runs clear, and it will take a while. What sand are you going to use? i recommend argos playsand, it looks really good and is really cheap (3 or ?4 per bag), but one bag wont be enough for your tank, im sure it says how much in the catalogue. good luck!
edit: it'll be less stressful for the fish if you remove them from the tank whilst changing substrate, but wash the sand first before taking the fish out, because you want them out for as little time as possible.