This is much simpler than most related articles make it (and good luck finding it in Amazon’s own documentation).
- Name your bucket the url you want to use for the cname.
- Set up a cname to s3.amazonaws.com.
- 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.