The main difference with previous versions is the way to package your application, in the previous versions you had to change the content of the file portal.war in order to customize the portal. Now we more consider your application as an add-on that you can packaged in another ear/war file. You just need to follow some rules in order to notify the platform that it must take into account your add-on in order to customize the portal.
Among other things, you will have to:
Indicate the platform how to initialize and manage your application by defining and registering the PortalContainerDefinition related to your portal instance.
Deploy the starter ear/war file that is used to create and start all the portals (i.e. portal containers).
Please take into account, that you need to ensure that the starter is launched after all the other ear/war files.
If you don't need to customize the portal, you don't have to deploy the starter. The old way to deploy an application is still compatible.