If it's the Flourite red it's real dusty. If it's the normal Flourite it's still a bit dusty. The way I do it is put about 1/3-1/2 the bag in a five gallon bucket. Then take it out to the yard and use the hose to rinse it. Stir it around a bit but don't get too aggressive with it. Then carefully pour the dirty water out while keeping the majority of the 'fines'. Do this a couple, three times till the water is fairly clear and then put it in your tank. When you are finished put a plate in the tank and fill it slowly directing the water flow over the plate.
The other hint is to put a thin layer of ground peat at the very bottom. This will quickly create an acidic environment to allow the plant roots to access the iron in the Flourite.