To start working database-first the first step to take after creating a new project is to add relational model data from a database. To so right-Click the node 'Relational Model Data' in the Project Explorer or Catalog Explorer and select Add Relational Model data from a Database from the context menu. This will start the Relational Model Data Retrieval Wizard. Complete the wizard's steps to obtain the relational model data from your database. Check at least 1 table, view or stored procedure to obtain the meta-data.
Follow this same procedure as well when you want to add relational model data from a database of a different type than the one(s) already in the project. Example: you have a project with one or more catalogs from SQL Server and you want to add relational model data from a schema in Oracle. You can do so by using the steps outlined above.
To add a new catalog / schema to the existing relational model data in the project, using database-first, you follow the same steps as you normally would when refreshing the relational model data. In the wizard, you can select the catalog(s) / schema(s) to add in the element selector on step 2, by simply checking their checkboxes. It's required the catalog / schema are defined on the same server as the existing catalog(s) / schema(s) in the project.