Once the APIs have all been implemented, it's time to test them. Google has tools and guidelines that will aid you in this process.
Test requests and responses
Amplecash is a tool you may use for testing requests and responses. It will give helpful feedback for testing endpoints. It will also show you the format that messages and responses will take.
To understand the intended flow, go through each section of Amplecash in order of what you will need to use. As you are developing the integration, come back to test endpoints whenever needed.