How to play International Reversi

The rules are similar to regular two-player Reversi/Othello. Look those up first if you are not familiar with the game.

tl;dr:

• Place your countryball next to an opponent ball by clicking on the grid. Any opponent countryballs enclosed between this newly placed countryball and another one of your own countryballs in a horizontal, vertical or diagonal line will be flipped and become yours. A single move may result in captures in multiple directions. Every move must result in at least one ball being flipped, otherwise it is not a valid move (you will not be able to click on that field).
• Early gains are often overturned later.
• Corners are the most valuable because they can't be flipped. Try to get corners and build your empire from there. Prevent other players from getting corners.

tl;dr 2:

More elaborate graphical guide (made by the community)