Place one stone on any empty cell.
Then every cell where your stones fill more than half of its neighbours becomes yours:
empty cells get your stone, enemy stones turn over. Repeat until nothing changes.
Needed: 4 of 6 inside, 3 of 4 on an edge, 2 of 3 in a corner.
Board full: more stones wins.
Swap: after the first stone, the other player may take it over instead of moving.