Please include inside http://developer.yahoo.com/oauth/guide/ a full example of using an access token with url (e.g. contacts request), base string, signing key, computed signature, and Authentication header. It seems many people are struggling through the encoding. An example like this which people can plug the example values their own code computes the correct signature and properly orders the Authentication header would go a very long way.
There's no need for the consumer key, nonce, timestamp, etc. to be valid, just that the computation works and the ordering shows correctly.