After creating a certain amount of entities you may want to create also relations between them. To do so:
Select the two, or more, entities you need (SHIFT button held down); right click; choose the menu item Add relation, and then select the type of relation you want. The entities will be linked accordingly.
As for the relation types ancestor and descendant, remember that: if you select first the ancestor, and then the descendant, you have to choose a descendant relation type. By comparison, if you do it the other way round, i.e. first descendant and then ancestor, you have to choose an ancestor relation type.
If you decide to remove some of the relations just created, select again how many entities you need (SHIFT button held down) and right click; choose the menu item Remove Relations and then Remove relations between selected. The line linking the entities will go away.