When adding sand myself i use a large bucket to wash it thoroughly, i do this by placing a hose pipe in with it, and leaving it to run for an hour or so, mixing it up every 10-15 minutes until it runs clear.
Then when adding it to the tank to minimise the disturbance, i use a pint glass and scoop it out of the bucket, then i place it in the tank and let it fill with water, and slowly move it to the bottom, it pours out slowly and gently.
Also if your gravel is fine enough it might be possible to slowly syphon it out using a hose pipe.