I had added some by using a 2" pipe .long enough to stick out the water,and put the other end where you want the sand .i used a small funnel and add the sand down the pipe .when you lift up the pipe the sand will flow out .so your tank won't get that cloudy.
If you just out it in a solo or other plastic cup and lower the cup slowly and turn the cup on its side. Then you can flex the cup and pour the sand out.