The general principles of using external .as files are simple. To associate a .as file with your main timeline, set the Document Class property of your fla. Your document class .as must extend Sprite or MovieClip. Usually MovieClip, if you have any frames or timeline coding at all (even comments).

To associate a symbol in your library with a class, set it to Export for Actionscript, and set the class name to match the desired .as class.

You'll want to learn how packaging and imports work, too.