In-app billing and in-app purchase

I am searching in the application on developer.android.com, but it is useless. I also installed and looked into my sample application, but I still don't understand what to do. because they use some undocumented methods to start the service.

I just need a simple and clean implementation, as a step-by-step guide. Please help me.

I still don't understand why they were too complicated for the developers? although integrating another API just adds a jar and calls some functions, for example. PayPal

Help me please. give a link to any good tutorial.