How to setup a custom or scoped UPM registry mirroring paid asset store packages?

In our company, we purchased some asset store assets with a "per-site" license. It would make sense to allow every member of our team to install those packages, but they are only available to the person that bought the package.

A great solution would be to create a custom (or scoped) UPM registry, into which we copy the assets we purchased from the asset store. However, I can't seem to figure out how to do this.

The use-case of making a local copy of the UPM registry, is mentioned in the documentation (https://docs.unity3d.com/Manual/upm-scoped.html , under "Access packages in a closed network environment."). But there is no explanation how to implement this.

Can anybody give directions on how to set this up?