This is much simpler than most related articles make it (and good luck finding it in Amazon’s own documentation).

1. Name your bucket the url you want to use for the cname.
2. Set up a cname to s3.amazonaws.com.
3. Grant the “everyone” role “view permissions” for the bucket.

For example, I created a bucket called assets.scottw.com. I then entered a cname record for assets and pointed it to the url s3.amazonaws.com. Now any file I upload to the assets.scottw.com bucket is available at the same corresponding url.