Hard-shell clams or "quahogs" like large chowder clams, littlenecks, and cherrystones should have tightly closed shells. If you can separate the shells easily, then the clam inside is dead. Clams that are already dead should not be purchased or, if already purchased, should be discarded. Hold one clam in the palm of your non-dominant hand. Use your dominant hand to pass the blade of a small, dull knife in between the two closed shell halves.  Work with on clam at a time. Hold the clam with a towel to prevent juice from dripping onto your hand and to provide extra traction as you cut. The knife should have a blade that is thin enough to pass in between two tightly closed shell halves, but the blade should also be fairly blunt to minimize the risk of accidentally cutting yourself. Use a shucking knife if possible. Slide the knife around the perimeter of the clam to loosen both the adductor muscles and hinge.  Close your hand around the clam as you work, squeezing it slightly to put pressure on it. Note that it will likely be difficult to pass the knife around the perimeter of the clam, and you may have to press or squeeze the knife in between the two shell halves. Twist the knife, rotating it from side to side, until the top shell lifts off.  Sever the top shell completely by cutting the upper adductor muscles with the blade of your knife. Lift off the top shell. Slide the knife below the exposed crab meat, severing the lower adductor muscles. Do not lift the crab meat out of the bottom shell yet. Fill a shall baking pan with ice and lay the shucked clams on top of the ice until you are ready to add them to the chowder. Place the clams in the ice shell-side down.

TL;DR in Chinese:
购买新鲜的硬壳蛤蜊 硬壳蛤蜊或圆蛤类包括大蛤蜊，小帘蛤以及小圆蛤，都有紧紧合上的外壳。 用一把刀插进蛤蜊的两半壳里。 切断连接处。 将上面的壳撬开。 取出蛤蜊肉。 使用之前先将蛤蜊放在一旁。