Best way to generate Feed

I am trying to generate feeds so that visitors can subscribe to them.

What is the best way to each such task. I gave a try at AtomFeedHelper,
but the documentation is not very clear, and I don't find I can tweak as
I want. Also it gave some errors when validating at the w3c.

I saw Ryan Bates at railscasts has something for generating rss.

Do you have an tips or about that?