1. Right click on an open project in the Projects navigator and select Add New... (The New File wizard opens).
2. Select C++ under Files and Classes and C++ Class in the second list view. Click the Choose button. (The C++ Class wizard opens)
3. Enter a Class name, select <Custom> base class and enter an existing class name to inherit from.

It would be nice to have an "Add Q_OBJECT" checkbox. Then when the wizard completes, the Q_OBJECT macro is added to the header file appropriately.