This was one technical step in a pre-sales date, so I am not sure if I remember everything.
The system was already configured to get updates from Amazon so yum configuration already existed and was working fine. reposync allows to repository locally based on the yum configuration, we added the options --downloadcomps --download-metadata --newest-only to reduce size but get all metadata. We then recognized that errata metadata are missing as reposync did not seem to handle them, but they were stored to /var/cache/yum and with modifyrepo_c allowing to add metadata we took them from the cache and added them to the local repository. Not sure if a createrepo_c was required first, but I think the initial metadata were provided by reposync.
With this we got the repository to Katello and then the customer wanted to test deployment and subscription-manager, as he reported no error I think it worked without any hassle, but I do not know.
Hope this helps at least a bit. If you get it to work, feel free to write a short tutorial. If not, I will try to help but I have no access to Amazon Linux at the moment (and no time) to work through it completely.