Put the oyster opener on a table so that it rests against the edge of the table. Put the oyster into the opener, with the pointed end facing you. Insert the knife into the pointed end of the oyster.
Swing the knife round from side to side, keeping the point inserted into the end of the oyster. This will cut through the hinge of the oyster - be forceful!
When the hinge is cut through you can then use the knife to follow round from the point, easing the oyster open.
Then remove the top shell, lay the bottom shell with the oyster in it carefully onto a serving plate (they are often served on a bed of crushed ice). Make sure you keep as much as possible of the liquid in the shell.