What's important in a 24 mark essay is to clearly put forward your arguments for and against. This way, an examiner can see that you've fully understood the question and can respond to it in an organised and coherent manner.
For example, if your question was "To what extent was Cleopatra successful monarch?" (obviously the questions would never be this easy, but we'll go with it!) you'd make a mini plan plotting out the arguments. In the for section, you'd use points such as her long lasting reign to agree with the question. You'd need evidence to back up your claims, usually taken from what you've been studying but also from extra research. You'd make roughly 3 points for, then switch to the against side. It's the same format, but disagreeing with the question instead. Using our examplar, you could say that Cleopatra was unsuccessful because she often relied on the power of Rome, rather than making her own way.
After you've made these arguments, it's important to come to a strong and reasoned conclusion. This way an examiner can see that you've absorbed the information and understand it enough to make your own judgement about it.