There are a lot of good methods for this.. I usually just take a piece of about 3" PVC pipe that I cut and add the sand through it. The lower end of the PVC sits on the bottom, and the top is out of the water. I use a funnel to get the sand into the PVC. The sand fills up at the bottom of the PVC; and then you can just slowly lift the PVC off the bottom and the sand slowly rolls out on the bottom of the tank.
It's important to really rinse the sand, depending on the type of sand you're using. Then when you add it wet it's less likely to fly around the tank like a snow globe.