To easily create a subtype which inherits from a nominalised fact type (or object type), the menu item from the fact type context menu can be used. It contains a menu item called "Create new subtype".
The dialog which appears looks as follows:
You should enter a name for the subtype, and select any roles which are to be connected to this new subtype. (Before Modeler v8.8 a minimum of 1 role is mandatory.)