The technical aspects are described in sections “Retrieving objects” and “Serving objects” of ap-next guide: https://codeberg.org/ap-next/ap-next/src/branch/main/guide.md#serving-objects
Setting wrong headers is the most common mistake.
One additional thing that is currently not stated in the guide and might not be obvious is that one URL may resolve to either an HTML page or an AP object depending on the value of Accept header. I’ll probably add this information to the guide.