Flip it upside down, dunk it, and then turn it over when it's under water.I use a pipette because if you just dunk the vial I'm afraid you get too much of the surface that fills in as you dunk it, so any surface oils or things will fill the vial. I pippette specifically from under the water line by the power head. Probably doesn't make a difference, just want to make sure I get a clean sample of water.