A LETTER/AN EMAIL (formal or informal) is written in response to the situation outlined in the task. Letters/emails in the Cambridge English: Advanced Writing paper will require a response which is consistently appropriate for the specified target reader.
When a response is framed as an email, letter-writing conventions such as an opening salutation, clear paragraphing and closing phrasing are always important.
DO begin by giving a reason to write. You would normally start with a greeting, then acknowledge the letter or some key information given in the letter to which you are replying.
Use paragraphs in which you cover each of the points mentioned in the task input.
As well as the points mentioned in the task input, think of some of your own ideas.
Identify the function(s) you should use (e.g. advising, reminding, requesting, suggesting…).
Make sure the points covered follow a logical right order so that the whole letter/email is coherent.
Finish the letter/email in a natural way, by arranging to see or contact the person your are writing to again soon.