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Drop supply lines into opening. The rubber washer goes between the faucet and counter to give a watertight seal.
Put the metal washer on underneath, and tighten the locknut with a basin wrench. Connect the cold and hot water supplies to the shut-offs. Before testing the water, remove the screen on the faucet. If there is any sediment in the line, it would clog the screen.
Roll a ball of plumber's putty into about a 1/4" rope and place it on the underside of the drain basket. Set the basket in the opening and the gasket and washer underneath, and tighten the pieces together.