Newer Mercury two stroke motors have oil injection systems that keep the ratio of gasoline and oil correct. Older two stroke motors from Mercury do not have those systems, thus, the oil and fuel need to be mixed by hand. This is a task that is not too difficult.
Start by gathering an empty, clean gasoline container and two cycle outboard oil. Next, look at how much gasoline you pumped into the container. For each gallon of gasoline in the container add three ounces of motor oil. Close the container and shake the contents vigorously before adding the gasoline and oil mixture into the fuel tank.