At first step you should deploy your fleet on the battlefield.
All your ships are already placed there in random positions. You can move them as you like by dragging them and rotate them with double click. You can generate another random disposition also by clicking Autodeploy button. You may also select a difficulty level there.
When you are ready, click the Fight! button. You will see your fleet placed on your field on the left side and empty field (because you know nothing about it for the present) of your opponent on the right side.
It's your turn now. Select the cell on the opponent's field and click it with left mouse button. Bombs away!
You and the other player (computer) bomb one cell of opponent's plot in turn. Opponent responds by telling you've "missed", "hit" or "sunk" his ship. If you've not missed, opponent skips his turn and you may bomb again. The winner is the first to sink all enemy's ships.