I want to learn how to write an appropriate letter to the editor in no more than 200 words.

I took the article "stuck in the middle" by Jeevan Vasagar of "The Guardian" as an exercise, you can find it on Google. Unfortunately, I am not allowed to post websites.

Can anyone help me ? Thanks in advance.

There are several good websites in here to help you learn what to include and how to write a letter to the editor:

http://www.google.com/search?q=how+to+write+letter+to+the+editor&aq=f&oq=how+to+write+letter+to+the+editor&aqs=chrome.0.57j0l3j62l2.5941j0&sourceid=chrome&ie=UTF-8

And is this the article you are referring to?
http://www.guardian.co.uk/uk/2001/may/23/immigration.immigrationandpublicservices4

What word processing program do you use? Word? It has a word-count feature in it so you can deal with the 200-word limit.