Suppose that Croatia and Liechtenstein both produce olive oil and glass. Croatia's opportunity cost of producing a pane of glass is 5 crates of olive oil while Liechtenstein's opportunity cost of producing a pane of glass is 11 crates of olive oil.
By comparing the opportunity cost of producing glass in the two countries, you can tell that has a comparative advantage in the production of glass and has a comparative advantage in the production of olive oil.