Paul Decarlo Has written an amazing blog on how to use the App Studio web app template.
"Microsoft’s App Studio portal allows developers to build rapid prototypes as well as full blown applications quickly and easily on any machine with a capable web browser. This has resulted in thousands of published apps from developers around the world! In today’s post, I am going to focus on one of the many provided templates, the Web App Template.
The Web App Template started as a project on Codeplex that allows for the creation of an app experience on Windows Phone and Windows 8 by leveraging your existing website. This approach comes with various pros and cons, namely that the more responsive design implemented in the original website the better the layout is when wrapped into an application, in addition the app requires accessibility to the internet for full functionality. However, this can be seen as a great opportunity for developers looking to create applications which require little maintenance to render across multiple platforms i.e. Android, IOS, Windows etc."
To see the full 25 step walkthrough visit his blog post here: http://pjdecarlo.com/2014/08/using-web-app-template-in-app-studio-to-publish-top-music-videos-mobile-wp8-in-25-steps.html