It would have been a lot better had you put the water in first, so you could easily stir the salt into the water while it is in the tank.
You now have two options. You can still stir it while it is in your tank, but the water is going to get incredibly cloudy. I recommend mixing the water and salt in a large plastic container. Then put a large plate inside the tank and carefully pour the mixed saltwater onto the plate so that you disturb the sand as little as possible.