I fid a funnel has faults as once the funnel is wet inside, the sand sticks to that. I use a DIY soda bottle.
I cut the bottom out of a 2 liter if it is for a big area or smaller for small tanks. Leave the cap loosely in place, fill with sand and gently push the bottle , neck down, into the water. Make sure to go slow as the water runs over into the overturned bottle, so that it doesn't stir the sand too much. When you have the cap nearly down into the existing sub, use a couple fingers to remove the cap. I find my cap floats up and away so that it is easy to find later. With the top open, I can release the sand as fast or slow as needed by moving the top higher or lower as I move around among the plants. With the new sand fully in the water, it doesn't stick to the sides.
Lowering the tank water is also helpful in deep tanks.
One note of caution when removing the empty bottle, pull it up slowly to avoid the rush of water out the top from blowing sand around.