It's not yet clear (at least to the public) how the registration will actually work on a technical level. I think the idea is that there will be third party services who are responsible for validating the identity of a user (similar to what banks use for KYC purposes). Users could identify themselves via different means, e.g. by providing their phone number or by simply uploading an ID. But that's all speculation on my side.
The easiest way is when the user provides their phone number and confirms it by receiving a SMS. That's how users of public WiFi points are identified in Russia and how users of IM programs are supposed to be identified in near future.
