my to do it from scratch?
* create MicroStation / Revit / Civil files *is there special settings?)
* create a new Project (inside connect.bentley.com) from scratch
* what should be done inside this project? enable iTiwins?
* how to upload files and related changes?
* how to see that files inside Project ?
There isn't anything special that you would need to do with your design data. You can create a new project or if you have an existing one that you want to use, you can certainly do that as well.
As far as preparation to the project, you'll need to create a blank iModel in the iModel Manager for that project.
Once you have that setup, you would then need to create a new Synchronization in the iTwin Synchronizer. You would setup which file(s) you want to synchronize and select the iModel that you've created for the Project. Then you would process to Save the Synchronization and then Synchronize your changes.
After the Synchronization is done, you can then view the iModel in the Project. If you created a Named Version, you can then see the elements.
Let me know if this answers your questions.