Each player has a grid of their own and places a fleet on it. Players then fire at the opponent's grid: a shot either misses or hits a ship. A hit earns another shot, and the turn passes to the opponent only on a miss. When every square of a ship has been hit, the ship is sunk. The first player to sink the opponent's whole fleet wins.
The fleet has one ship of four squares, two of three, three of two and four of one, and ships may not touch each other, not even at a corner. So there is never a ship diagonally next to a hit: fire above, below, left or right of it, and once a second hit shows which way the ship lies, keep going along that line. When searching, fire sparsely at first, say on every other square like a chessboard, because that finds the big ships with fewer shots. The single-square ships are left in the empty gaps at the end.