To make a generated NWSync manifest playable in the game client, you need to generate it with the flag
--with-module --mod-uuid <UUID>.
Give the module a UUID that you generate locally. All future versions of the module need to match the same UUID to be considered part of the same module in the UI.
Generating the data --with-module ensures that all serverside content is also packaged up inside the manifest; otherwise, only content needed for clients to play on dedicated servers would be part of it.
The game queries ROOTURL/repository.json to index a module repo.
For the official curated repository, see http://curated.nwn.beamdog.net/repository.json. This is a good starting point to see all supported features.
Any assets referenced (such as screenshots) can be placed anywhere within the repository root. Some organisation (e.g. by UUID directories) is recommended.