You need to prepare by improving your general writing skills.
Make sure that you:
Know, in general, the appropriate format and style for a standard shot message, business letter and business report.
Know how to select the appropriate information to be included in your document
Know how to structure the information in clear and logical sections
Know how to write in a style appropriate to the type of document and reader
Know how to use a range of basic grammar and standard phrases accurately
Keep within the time frame for the task