Have you read the document checklist at all?
If not, start by doing that. >> http://www.cic.gc.ca/english/pdf/kits/forms/IMM5610E.pdf
Look under number 10 for the content requirements.
Then use this template to write up the letter
>> http://cectoolkit.weebly.com/refletter-template.html
Keep in mind that this is a general template and has to be modified.