A student asked me this useful question:
In writing task 2, isn't it better to write about just one idea in each main body paragraph? If we put two or three ideas in the same paragraph (e.g. firstly, secondly, finally), we can't explain each idea in as much detail.
Here's my answer to this question:
Some questions require you to put more than one idea in each main paragraph. For example, if the question asks you to explain the advantages, disadvantages, reasons, causes or solutions, you must give more than one. Therefore a "firstly, secondly, finally" paragraph structure is perfect. If the question doesn't contain a plural word, one idea per paragraph is fine.
To learn the two methods that I use for writing paragraphs, study this lesson.