Right click on the body of the form.
Click Change Signals / Slots
Under slots, press the + key. Add the name of the callback that you want.
Link it with the signal/slot editor
Now your generated file of the same name will have a callback of the name of the slot you added
Derive from your generated file
Use public slots: to implement, as per the manual
—EDIT—
Found an easier way
Derive from the autogenerated class
Write something like
public slots:
void on_actionSQL_Query_activated();
Replace actionSQL with the name of your object. Replace activated with the name of the signal that is sent.