There are some general guidelines that you can follow, but you would do well to imagine the person who is sitting in an office having to read all these letters! What is really important is that you write YOUR letter TO THIS PERSON ABOUT THEIR COURSE. Do some research and find about their course. What modules on the course interest you? How would these specific modules help you achieve your goals? You need to show the reader that you have done some research into their course and that you have thought about how these modules are relevant to you and your ambitions. If the reader thinks that you are writing the same letter to lots of other institutions they will not be impressed! They will not be impressed if they think that you are desperate to do ANY course just so that you can go home with a certificate! Do check and edit your letter carefully but - it doesn’t need to be perfect! It is from you, a non-native English speaker, and plenty of non-native English speakers get good degrees without perfect English. In fact if it sounds too good it may sound like it’s not from you at all!