You could be asked to write a formal or informal letter as the first question in either Paper 2 or Paper 3.
Remember, in Paper 2 the examiners are predominantly looking for your reading skills, so you are awarded more marks for the content of your writing- up 15 marks are available. You can also be awarded up to 5 marks for the quality of your writing.
In Paper 3 the examiners are looking for your writing skills. You can be awarded up to 15 marks for your writing skills and 10 for your reading.
This video focuses on a past paper which you have read before. June 2010 Paper 2 0500/22.