To remember what the main characteristics that differentiate classical and modern liberals, divide their differences according to their attitudes towards the main ideological beliefs that define liberalism. These include the views on freedom, individualism, the role of the state, economics, society, etc. For example, if you were talking about individualism, the main differences for this would be that classical liberals believe in egotistical individualism and that modern liberals believe in developmental individualism. Go on to define these terms and the implications these views have for other dividing beliefs, such as the role of the state, for example.
You could also consider in your essay question whether these differences mean that liberalism is inherently divided as an ideology or united to show that you have a more in-depth understanding of the topic.