We have all the same goal in mind, but Apple does its best to keep this goal as far and impossible as it could be 🙂
I'm afraid you have to work through all the docs and threads that are out there, all the time.
Embarcadero has a quite good explanation of the whole process for the start, but it may break easily at every little step in every little building block involved (MemberCenter certificates/provisioning, key chain old/double provisioning files, Macos setup, Macos updates, Macos version, Platform CPU, iOS version, iOS/Macos off-sync. versions, XCode version, SDK-Versions, SDK-Missing internal parts, network/wifi connection, Transporter version, AppStore, certificate, DeviceID, Device image, PAServer, Delphi version, ...).
Even XCode itself sometimes cannot resolve everything smoothly, you will have to dive deep into all those forums.
It would help if you would present a failure message you've got, to get more insights.
Usually the best way is to prepare a new ID, certificates, devices in the MemberCenter manually and to use XCode to prepare iOS device for development, starting an empty project on iOS, it can manage provisioning files to some degree.
Once it works, usually it stays quite stable for a long time.
When you leave it alone for two or three weeks, you might see some provisioning issues and need to re-establish the connection sometimes (at least I have such experience here).