Qt SDK was a good way to start. For me things evolved in the following sequence:

* I got familiar with the Qt framework/libraries for C++ by looking at tutorials. In addition to some specific concepts like signals and slots.
* Take a look at QML which is the recommended way of making the GUI elements of your application (you could even code all your logic in QML or javascript if it was a light application)
* Get familiar with the Qtquick components for meego/harmattan. They are specific QML elements ready made for the N9 and they will make your application have the same feel as others on the device.
* Practice makes perfect (have fun)