As for the back end, if you use the advice from @Roc Martí you should be OK.
When designing the interface, it is kind of difficult to ignore the differences between a 19-inch monitor and a mobile's monitor and its orientation change feature, so I guess some attention is required for the GUI, however, that totally depends on your application of course.
As for the steps you mentioned, I find that they are not in line with any of the methodologies I am aware of. For example, there is no Planning or Analysis or Requirements determination. Also, there is no Deployment/Production activities listed. So, be careful.