I have no idea what you are trying to do based on your question, but since the title is 'external style sheet', I'm guessing. An external stylesheet is a .css file. It can be created in Notepad, maybe that's where the confusion with the new text doc comes from - just make sure to save it as whatever.css. At times, however, your computer may add an extra .txt to the end that may not even be visible. Changing your settings to show hidden file types will help.
See this post for details:
http://www.killersites.com/forums/post/5560/#p5560