[If you do not already have a project-based diagram open (see section 2.1 above - option 'Open Diagram')] From the File menu, select New Diagram of Type and then Query Diagram. As the name says, this diagram is mainly used when you want to search for entities within the ones present in the database.
What you can do here is either type something for yourself, or directly use the entities in the tab Project Tree on the left of the page.
In the first case, you can type something like 'x[Albert_I]' in the tab 'Kin Type Strings' on top of the page: in the main panel you will see all the entities named Albert_I and/or containing Albert_I in their names. Besides, you can add futher kin types like 'x[Albert_I]*', in which case you will see all the possibly existing first-degree relatives of all the entities present in the main panel.
In the second case, by comparison, you can select an entity in particular from the 'Prohect Tree' tab and drag it to the Kin Type Strings tab on top of the page, preceded either by E or x, which stand for any, unspecified entity [note that if you want to narrow the field of search you can always open the 'Search Entities' tab via 'Panels' menu]. E.g. x[Entity.Identifier=7bd7bff7-ff5e-4424-a9c9-d1283e0df145]; in the main panel you will see only that entity in particular. As in the previous case, you can also add further kin types, and the diagram in the main panel will change accordingly.