Peter,
The gluing as you call it happens within the Agentry Editor.
1. The open UI components (Java code) you create must be compiled into the Agentry client using the process defined by the Open UI SDK. For Android, you need to setup the various projects as described in the SDK documentation. All your code going into the AgentryAndroidClientSolution project within the com.sap.mobile.platform.client.openui.extensions package. The Agentry client must then be compiled which will include the base client along with your custom code together.
2. Within the Agentry Editor you can then set your fields to utilize your open ui classes instead of the base classes for the fields. When you run your application using your compiled client it will then display your open ui controls.
3. You can extend only one or as many of the base control classes as you want. You can create multiple variants of the same base control as well within one Agentry open ui compiled client. You determine which control is used for each field within the editor by specifying the class name you created.
I would start with compiling an open ui client with just one open ui extended control. Start simple and then built up to more complex controls.
Have fun!
--Bill